String tokens used outside stop functions aren't migrated to expressions

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**mapbox-gl-js version**: Latest

**browser**: Chrome

### Steps to Trigger Behavior

1. Create a new style in Studio
2. Replace the text-field of the `poi-label` layer (or any other Streets layer) with a string token `{name}` used in your style.json
2. Upload the style to Studio
3. Open the style and see the `{name}` string token persisting as the text-field for the `poi-labels` layer

[Copiable link](https://api.mapbox.com/styles/v1/kelsey-taylor/cks9eqaai013m17t2x8smuil3.html?fresh=true&title=copy&access_token=pk.eyJ1Ijoia2Vsc2V5LXRheWxvciIsImEiOiJjazJ3ZWl2a2YwZHU2M21xbDJnYXA2Z29tIn0.pnZVgZveufjFh20yW0ZI7g) to style

### Expected Behavior

`{name}` should be converted into `["to-string", ["get", "name"]]` via the expression migrator

### Actual Behavior

per @samanpwbb, string tokens are only migrated if they're not on the root of the style: https://github.com/mapbox/mapbox-gl-js/blob/dc8a6308ce3f072b6d87ff556721d1401b8a1f5b/src/style-spec/function/convert.js#L246-L269 https://github.com/mapbox/mapbox-gl-js/blob/dc8a6308ce3f072b6d87ff556721d1401b8a1f5b/src/style-spec/function/convert.js#L24-L26

Start in src/style-spec/function/convert.js at the referenced lines and trace why string tokens on the style root bypass migration. Add coverage for a root-level {name} token and verify that it becomes ["to-string", ["get", "name"]] through the expression migrator.

Written by the indexing model from the issue text.