Coerce strings to colors automatically in expressions

Nobody has claimed this yet.

For some reason, the `'#ffff2f'` is not valid and it now requires the `to-color` conversion. Is this expected?

```js
paint: {
// does not work
// "line-color": [
// 'at',
// 2,
// ['literal', ['#d7191c', '#fdae31', '#ffff2f', '#a6d96a', '#1a9641']],
// ],

// works
"line-color": [
'to-color',
[
'at',
2,
['literal', ['#d7191c', '#fdae31', '#ffff2f', '#a6d96a', '#1a9641']],
]
],
},
```

Example: https://jsfiddle.net/NameFILIP/oLw1ysjm/42/

Start by reproducing the line-color expression from the linked JSFiddle and trace how the expression result is validated for color properties. No source files or tests are named in the issue, so identify the relevant expression and paint-property code first. Done means the expected handling of hexadecimal color strings is established and covered by the appropriate behavior or documentation.

Written by the indexing model from the issue text.