Add ZoomConstantExpression to StylePropertyExpression

[tangerine-orange]

Describe the bug
In this piece of code from maplibre-gl-gs:

    _handleSpecialPaintPropertyUpdate(name: string) {
        if (name === 'line-gradient') {
            const expression: ZoomConstantExpression<'source'> = (this._transitionablePaint._values['line-gradient'].value.expression as any);
            this.stepInterpolant = expression._styleExpression.expression instanceof Step;
            this.gradientVersion = (this.gradientVersion + 1) % Number.MAX_SAFE_INTEGER;
        }
    }

we are coercing expression to ZoomConstantExpression<'source'>. This is required because expression is defined as a StylePropertyExpression = ConstantExpression | SourceExpression | CameraExpression | CompositeExpression.

This coercion tells me that there's some problem with the typing here. I see two options:

1. expand StylePropertyExpression to include ZoomConstantExpression, and possibly ZoomDependentExpression as well. Currently, these are the only two expression types that StylePropertyExpression doesn't include.
2. In mapbox-gl-gs codebase, set expression: StylePropertyExpression | ZoomConstantExpression<'source'>

I'd think option 1 is better, but I'm very familiar with the types here. @birkskyum do you have an opinion?

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This issue came up when investigating maplibre/maplibre-gl-js#2837

[HarelM]

I think 1 is better, but we need to understand why it wasn't like that to begin with, i.e. is adding it to the type generalise it too much or is incorrect in some places...

[tangerine-orange]

agreed for sure. I will do some investigation, hoping that we will get some input from @birkskyum or someone else who knows.

[birkskyum]

I don't know this area of the library well unfortunately.

[tangerine-orange]

Looking at this some more, I don't know if adding ZoomConstantExpression and ZoomDependentExpression to StylePropertyExpression is the right move. The structure of the zoom expressions is a bit different than the other style expressions. expressions are pretty complicated, with lots of nested expressions and different types of expressions.

Rather than just adding the zoom expressions to the StylePropertyExpression definition, a more comprehensive look at the typings of expressions is needed here.

For now, I think we should just expose an isZoomExpression type guard like @HarelM suggests in maplibre/maplibre-gl-js#2837.

[HarelM]

Fixed by #334