Add support for FillStyleLayer

[hactar]

Issue/Motivation

This PR adds support for MGLFillStyleLayer via FillStyleLayer. I based this off of LineStyleLayer but adjusted the properties. Included the properties that I currently need for a project, the todo of adding them all still applies.

Tasklist

• Include tests (if applicable) and examples (new or edits)
• If there are any visual changes as a result, include before/after screenshots and/or videos
• Add #fixes with the issue number that this PR addresses
• Update any documentation for affected APIs
• Update the CHANGELOG

[ianthetechie]

Ooh, nice! I am also embarrased to realize that I have a stash with basically the exact same thing from September that I never finished 😅 Thanks for adding this.