Rework spec function/expression taxonomy (recreate #6505)

[lbud]

Recreates #6505 — I reverted until I have a gl-native PR ready, as it'll otherwise cause a lot of code generation errors for the next person who tries to update the gl-js pin there.

New since #6505 I've also added a list of property-types and descriptor docs in a75b518 [reverted]

---

This PR reworks the taxonomy to describe how a property can use functions/expressions, as described in #6389 (comment) and #6389 (comment):

type ExpressionType = 'data-driven' | 'cross-faded' | 'cross-faded-data-driven' | 'color-ramp' | 'data-constant' | 'constant';

type ExpressionParameters = Array<'zoom' | 'feature' | 'heatmap-density' | 'line-progress'>;

type ExpressionSpecification = {
    interpolated: boolean,
    parameters: ExpressionParameters
}

StylePropertySpecification = {
    ...
    'property-type': ExpressionType,
    'expression': ExpressionSpecification
}

so now any property all properties include a property-type key, and for all property-types except constant also an expression property, i.e.

    "type": "number",
    "property-type": "data-driven",
    "expression": {
        "interpolated": true,
        "parameters": [
            "zoom",
            "feature"
        ]
    }

Benchmarks: http://bl.ocks.org/lbud/raw/2ac3b8869074c112802166978cea8345/ (I couldn't reproduce the Layer discrepancies on subsequent isolated bench runs).

Closes #6389
Supersedes / closes #4194
Refs #6430
Refs #6303

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• post benchmark scores
• manually test the debug page

[lbud]

@samanpwbb @anandthakker @jfirebaugh @1ec5 requesting review from anyone based on the new additions to this since #6505 (noted at the top of OP) — format + language of the property-type docstrings added + style-spec docs page format.

[jfirebaugh]

I don't think we should add these to the documentation. We already have fairly complex terminology surrounding properties ("layout", "paint", "function", "expression", "interpolatable", "data expression", "camera expression", ...), and I don't think additional terminology is going to clarify the situation. Let's keep these types as an implementation detail.

[lbud]

I reverted the addition of property-types on the docs page in 5bd3e06. The only changes thereafter from the previous iteration of this PR (#6505) are related to changes in master since then that would otherwise cause tests in gl-native to fail when updating the gl-js pin.