fix(VDatePicker): weeknumber calculation refactored to fix bug

[abbierwolf]

Instead of calculating only the weeknumber for the first week shown and increment it, it is now
calcuted for every week.

fix #9764

Description

fixes #9764

Motivation and Context

#9764

How Has This Been Tested?

unit | visually

Markup:

<template>
  <v-container>
    <!--  -->
    <v-date-picker
      show-week
      first-day-of-week="1"
    ></v-date-picker>
  </v-container>
</template>

<script>
export default {
  data: () => ({
    //
  })
};
</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 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)

[johnleider]

Need to clean up the code styling. I'm surprised it passed lint. More information is located in our Coding Guidelines.

[abbierwolf]

I applied all coding guidelines I could find in the document. I could not find tslint or eslint configuration to make sure I found all the issues.

[abbierwolf]

@johnleider Travis CI had problems yesterday can you trigger a rebuild? Or is this not needed?

[jacekkarczmarczyk]

Could it be done without using the Date object? I remember it's having some problems on ios/osx and with different time zones.

Some magic numbers (like 604800000) could be turned into a constant with a meaningful name and written as a product of some better recognized values (for example 86400 = 24(hrs) * 60(min) * 60(src))

Last but not least method could be extracted to helpers so it could be tested wihtout involving all the component

[johnleider]

@jacekkarczmarczyk Please leave that in a review.

[johnleider]

Re-format this similar to:

return weekNumber(
  arg1,
  arg2,
  arg3,
)

[abbierwolf]

First argument is on its own line

[johnleider]

This was meant for line 69