Add enum for layer position

[julianrex]

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• Briefly describe the changes in this PR.
• Include before/after visuals or gifs if this PR includes visual changes.
• Write tests for all new functionality. If tests were not written, please explain why.
• Add example if relevant.
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Summary of changes

This PR adds a LayerPosition enum that shadows the underlying MapboxCoreMaps.LayerPosition struct to improve the Swift ergonomics of function calls.

For example, the layerPosition parameter in the following function call

mapView.style.addCustomLayer(
    withId: "Custom",
    layerHost: self,
    layerPosition: LayerPosition(above: nil, below: "waterway-label", at: nil))

becomes

layerPosition: .below("waterway-label"))

[julianrex]

cc @tobrun

[neelmistry94]

I feel like we need another test that specifically adds an annotation above location indicator Layer or vice-versa. And then we can get the stack and check for the order there as the assertion. That was a specific use case and would be an impactful test.

Further, this PR is going to warrant a Migration Guide change. Can we make that and link that PR as part of this ticket somewhere.

[julianrex]

Can we address this with upcoming annotation changes? /cc @nishant-karajgikar?