fix(VIcon): allow colons to be used in icon names

[glen-84]

Description

Implements the suggestion by @nekosaur in #13975 – match the start of an icon string with the registered icon sets, instead of assuming that any string containing a colon includes an icon set name.

Fixes #13975. (hopefully – not yet tested [see below])

Motivation and Context

See #13975. It allows icons to be URLs.

How Has This Been Tested?

There is no test file for the icon composable. I tried to add one, but I got a bit stuck:

// Composables
import { useIcon } from '../icons'

describe('icons.ts', () => {
  it.each([
    // Default icon set.
    ["success", {component: ???, icon: "success"}],
    ["https://example.com/icon.svg", {component: ???, icon: "https://example.com/icon.svg"}],
    // MDI icon set.
    ["mdi:success", {component: ???, icon: "success"}],
    // Custom icon set.
    ["custom:https://example.com/icon.png", {component: ???, icon: "https://example.com/icon.png"}],
  ])('should return the correct icon set and name', (props, expected) => {
    const { iconData } = useIcon(props)

    expect(iconData.value.component).toEqual(expected.component)
    expect(iconData.value.icon).toEqual(expected.icon)
  })
})

I'm not sure how to configure/reference the icon set components.

Markup:

// Paste your FULL Playground.vue here

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 backwards compatible changes and next for non-backwards compatible 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)

[nekosaur]

Here's how you could test

// Composables
import { defineComponent } from 'vue'
import { mount } from '@vue/test-utils'
import { createVuetify } from '@/framework'
import { useIcon } from '../icons'

declare const expect: jest.Expect
declare const it: jest.It

describe('icons', () => {
  const Component = defineComponent({
    props: {
      icon: String,
    },
    setup (props) {
      const { iconData } = useIcon(props)

      return () => iconData.value.icon
    },
  })

  const vuetify = createVuetify({
    icons: {
      defaultSet: 'mdi',
      sets: {
        custom: {
          component: () => null,
        },
      },
    },
  })

  it.each([
    // Default icon set.
    ['success', 'success'],
    ['https://example.com/icon.svg', 'https://example.com/icon.svg'],
    // MDI icon set.
    ['mdi:success', 'success'],
    // Custom icon set.
    ['custom:https://example.com/icon.png', 'https://example.com/icon.png'],
  ])('should return the correct icon name', (icon, expected) => {
    const wrapper = mount(Component, {
      props: {
        icon,
      },
      global: {
        plugins: [vuetify],
      },
    })

    expect(wrapper.text()).toEqual(expected)
  })
})