Add support for unit in NumberFormat

[varna]

NumberFormat expression uses Intl.NumberFormat and shares the same name. I casually assumed that this expressions is a simple wrapper and tried using it like this:

      'text-field': [
        'number-format',
        ['get', 'area'],
        {
          style: 'unit',
          unit: 'hectare'
        }
      ]

But it didn't work. I would like to replace options object with an actual Intl.NumberFormat, but that would be a major update. I created this pull request that unit feature could be patched in with identical API as currency:

      'text-field': [
        'number-format',
        ['get', 'area'],
        {
          unit: 'hectare'
        }
      ]

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Add unit option to number-format expression (h/t [varna](https://github.com/varna))</changelog>

[CLAassistant]

All committers have signed the CLA.

[varna]

I feel ignored 🤣 Do I need to tag someone?

[SnailBones]

Many thanks for your contribution @varna! This will be a good feature to have and your code and test look great to me.