Fix identity functions for 'text-field' (i.e. 'formatted' type)

[ChrisLoer]

Fixes issue #7311.

An identity function for the text-field property would return nothing because the type of the input wouldn't match the 'formatted' type expected for that property.

This PR restores previous behavior by using the Formatted.fromString function.

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs
• post benchmark scores
• manually test the debug page
• tagged @mapbox/studio and/or @mapbox/maps-design if this PR includes style spec changes

cc @jplante @ryanhamley @mollymerp

[ryanhamley]

🎉

[ChrisLoer]

For posterity, my manual test was just to add this:

    map.addLayer({
        "id": "points",
        "type": "symbol",
        "source": "points",
        "layout": {
            "text-field": { "property": "name", "type": "identity" },
            "text-font": ["Open Sans Semibold", "Arial Unicode MS Bold"]
        }
    });