Port Image Expressions

[kkaefer]

In #8684, we introduced image expressions for determining whether an image exists. They can be used to display a fallback image in case an image can't be found.

Relevant PRs that need porting are:

• Image expression mapbox-gl-js#8684
• Make image operator compatible with styleimagemissing event mapbox-gl-js#8775
• Fix buggy image coercion so image names resolve correctly mapbox-gl-js#8839
• Avoid throwing styleimagemissing event for empty strings mapbox-gl-js#8840

/cc @ryanhamley