Document how to use formatNumber in JS and Vue

Background

See gitlab-org/frontend/rfcs#73 (closed)

TL;DR languages use different ways to show numbers, for example English uses commas as decimal separators (,), while Spanish uses periods (.). Our Vue UI often ignores this!

✍ What does this MR do?

As described in the documentation this change enables as easily format numbers in all of our Vue templates:

• In Vue templates

<script>
import { formatNumber } from '~/locale';

export default {
  //...
  methods: {
    // ...
    formatNumber,
  },
}
</script>
<template>
<div class="my-number">
  {{ formatNumber(10000) }} <!-- 10,000 -->
</div>
<div class="my-percent">
  {{ formatNumber(0.5,  { style: 'percent' }) }} <!-- 50% -->
</div>
</template>

It additionally documents formatNumber as the favored way to display numbers:

• In JavaScript

import { formatNumber } from '~/locale';

// Assuming "User Preferences > Language" is set to "English":

const tenThousand = formatNumber(10000); // "10,000" (uses comma as decimal symbol in English locale)
const fiftyPercent = formatNumber(0.5, { style: 'percent' }) // "50%" (other options are passed to toLocaleString)

Does this MR meet the acceptance criteria?

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