fix(VDataTable): make it easier to use custom-filter

[nekosaur]

Description

change custom-filter signature, added example of custom-filter prop usage and filtering on specific columns.

new signature is (value: any, search: string | null, item: any): boolean

Motivation and Context

early on in the data-table refactor I combined it and filter to make things a bit more straightforward. but I see now that the resulting customFilter function went the wrong way. Replacing it right now is not very straightforward (with a signature of (items: any[], search: string | null, headersWithCustomFilters: TableHeader[], headersWithoutCustomFilters: TableHeader[]))

How Has This Been Tested?

playground, doc example

Markup:

<template>
  <div>
    <v-data-table
      :headers="headers"
      :items="desserts"
      item-key="name"
      class="elevation-1"
      :search="search"
      :custom-filter="filterOnlyCapsText"
    >
      <template v-slot:top>
        <v-text-field v-model="search" label="Search (UPPER CASE ONLY)" class="mx-4"></v-text-field>
      </template>
      <template v-slot:body.append>
        <tr>
          <td></td>
          <td>
            <v-text-field v-model="calories" type="number" label="Less than"></v-text-field>
          </td>
          <td colspan="4"></td>
        </tr>
      </template>
    </v-data-table>
  </div>
</template>

<script>
  export default {
    data () {
      return {
        search: '',
        calories: '',
        desserts: [
          {
            name: 'Frozen Yogurt',
            calories: 159,
            fat: 6.0,
            carbs: 24,
            protein: 4.0,
            iron: '1%',
          },
          {
            name: 'Ice cream sandwich',
            calories: 237,
            fat: 9.0,
            carbs: 37,
            protein: 4.3,
            iron: '1%',
          },
          {
            name: 'Eclair',
            calories: 262,
            fat: 16.0,
            carbs: 23,
            protein: 6.0,
            iron: '7%',
          },
          {
            name: 'Cupcake',
            calories: 305,
            fat: 3.7,
            carbs: 67,
            protein: 4.3,
            iron: '8%',
          },
          {
            name: 'Gingerbread',
            calories: 356,
            fat: 16.0,
            carbs: 49,
            protein: 3.9,
            iron: '16%',
          },
          {
            name: 'Jelly bean',
            calories: 375,
            fat: 0.0,
            carbs: 94,
            protein: 0.0,
            iron: '0%',
          },
          {
            name: 'Lollipop',
            calories: 392,
            fat: 0.2,
            carbs: 98,
            protein: 0,
            iron: '2%',
          },
          {
            name: 'Honeycomb',
            calories: 408,
            fat: 3.2,
            carbs: 87,
            protein: 6.5,
            iron: '45%',
          },
          {
            name: 'Donut',
            calories: 452,
            fat: 25.0,
            carbs: 51,
            protein: 4.9,
            iron: '22%',
          },
          {
            name: 'KitKat',
            calories: 518,
            fat: 26.0,
            carbs: 65,
            protein: 7,
            iron: '6%',
          },
        ],
      }
    },
    computed: {
      headers () {
        return [
          {
            text: 'Dessert (100g serving)',
            align: 'left',
            sortable: false,
            value: 'name',
          },
          {
            text: 'Calories',
            value: 'calories',
            filter: value => {
              if (!this.calories) return true

              return value < parseInt(this.calories)
            },
          },
          { text: 'Fat (g)', value: 'fat' },
          { text: 'Carbs (g)', value: 'carbs' },
          { text: 'Protein (g)', value: 'protein' },
          { text: 'Iron (%)', value: 'iron' },
        ]
      },
    },
    methods: {
      filterOnlyCapsText (value, search, item) {
        return value != null &&
          search != null &&
          typeof value === 'string' &&
          value.toString().toLocaleUpperCase().indexOf(search) !== -1
      },
    },
  }
</script>

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and breaking changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)

[codecov]

Codecov Report

Merging #7885 into next will decrease coverage by <.01%.
The diff coverage is 70%.

@@            Coverage Diff             @@
##             next    #7885      +/-   ##
==========================================
- Coverage   85.66%   85.66%   -0.01%     
==========================================
  Files         333      333              
  Lines        9091     9090       -1     
  Branches     2413     2413              
==========================================
- Hits         7788     7787       -1     
  Misses       1208     1208              
  Partials       95       95

Impacted Files	Coverage Δ
...vuetify/src/components/VDataTable/mixins/header.ts	100% <ø> (ø)	⬆️
packages/vuetify/src/util/helpers.ts	92.27% <100%> (ø)	⬆️
...es/vuetify/src/components/VDataTable/VDataTable.ts	83.23% <62.5%> (-1.01%)	⬇️
packages/vuetify/src/components/VTabs/VTabs.ts	83.75% <0%> (ø)	⬆️
packages/vuetify/src/components/VSelect/VSelect.ts	94.13% <0%> (+0.36%)	⬆️
...ages/vuetify/src/components/VSnackbar/VSnackbar.ts	94.73% <0%> (+3.82%)	⬆️

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