Filter validation doesn't invalidate single boolean value

Nobody has claimed this yet.

It looks like single boolean values are passing validation but results in invalid GL JSON. 🤔 It's not immediately clear why. [`getType` should be accounting for it](https://github.com/mapbox/mapbox-gl-js/blob/922d7b38c0466181fa6f0cebbec1e013ae59d475/src/style-spec/validate/validate_filter.js#L25-L27).

**mapbox-gl-style-spec version**: `13.2.1`

### Steps to Trigger Behavior

```js
import validateFilter from '@mapbox/mapbox-gl-style-spec/validate/validate_filter';
import styleSpec from '@mapbox/mapbox-gl-style-spec/reference/v8.json';

const errors = validateFilter({
key: '',
style: { glyphs: 'fake' },
styleSpec,
value: false
});

// []

```

### Expected Behavior

An entry in the error array indicating the boolean value provided is an invalid filter value.

### Actual Behavior

An empty error array returns resulting in no error found.

Start in src/style-spec/validate/validate_filter.js at the getType logic linked by the issue, then reproduce the supplied validateFilter call with value: false. Update validation so the call returns an error for the boolean value, and verify that the error array is no longer empty.

Written by the indexing model from the issue text.