Can mapbox-gl-style-spec 'isExpression' tell 'Filter syntax' from 'expression'?

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**mapbox-gl-style-spec version**: 13.3.0

### Question

I am trying to use `expression.isExpression(exp)` to determine whether an `exp` is an `Expression`. If not I need to do some other logical operations. But while I am trying to pass an [`filter syntax`](https://www.mapbox.com/mapbox-gl-js/style-spec/#other-filter) to `isExpression()`(like `["==", "color", "blue"]`), it returns `true`. Ideally, for `filter syntax` I hope `isExpression` return false, then I can do another process to deal with `filter syntax`(The different is in expression if you want use property from tiles you should put the key after get, like `['get','key']` but in filter syntax you don't need `get`). Could this be a bug?

### Links to related documentation

[Filter (deprecated syntax)](https://www.mapbox.com/mapbox-gl-js/style-spec/#other-filter)

cc w/ @springmeyer

Start with mapbox-gl-style-spec 13.3.0's expression.isExpression entry point and compare its behavior with the linked filter syntax documentation. Determine whether distinguishing filter syntax from expressions is intended, then document or specify the expected behavior and add coverage if an implementation change is warranted.

Written by the indexing model from the issue text.