Add isZoomExpression

[tangerine-orange]

Addresses #267

In maplibre/maplibre-gl-js#2837, we need a way of checking if a given expression is a ZoomConstantExpression, and then casting it as such. In this PR, we expose an isZoomExpression type guard.

Note: I'm not sure if I'm meant to post a PR where I integrate a staged version of maplibre-style-spec into maplibre-gl-gs?

Launch Checklist

• [x ] Confirm your changes do not include backports from Mapbox projects (unless with compliant license) - if you are not sure about this, please ask!
• Briefly describe the changes in this PR.
• Link to related issues.
• Include before/after visuals or gifs if this PR includes visual changes.
• Write tests for all new functionality.
• Document any changes to public APIs.
• Post benchmark scores.
• Add an entry to CHANGELOG.md under the ## main section.

[HarelM]

This is generally approved, please check that it does what you would expect when using it in gl-js.

[HarelM]

I now read the linked issue comment saying you tested it to work as expect, cool, thanks!
Please create a version bump PR after this is merged so I can release a new version of style spec that will address the "real" issue.

[HarelM]

Oh, a changelog entry is missing, can you add it as part of the version bump?

[tangerine-orange]

Oh, a changelog entry is missing, can you add it as part of the version bump?

🤦 Ugh, I had a few local changes that I didn't commit 🤦, including the changelog entry, and those changes were what I was testing mapbox-gl-js against. I added both to #335.

[tangerine-orange]

Oh, a changelog entry is missing, can you add it as part of the version bump?

🤦 Ugh, I had a few local changes that I didn't commit 🤦, including the changelog entry, and those changes were what I was testing mapbox-gl-js against. I added both to #335.

I guess best practice is to use the pushed branch version of mapbox-style-spec to test, instead of using the local copy like I did.

[HarelM]

It is, but it's a chicken and egg problem as we wouldn't want to merge things that we're not sure are working...