Add loadStyleURI/loadStyleJSON

[julianrex]

Follows #352

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Pull request checklist:

• Briefly describe the changes in this PR.
• Write tests for all new functionality. If tests were not written, please explain why.
• Add example if relevant.
• Document any changes to public APIs.
• Apply changelog label ('breaking change', 'bug 🪲', 'build', 'docs', 'feature 🍏', 'performance ⚡', 'testing 💯') or use the label 'skip changelog'
• Add an entry inside this element for inclusion in the mapbox-maps-ios changelog: <changelog>Added MapboxMap.loadStyleURIandMapboxMap.loadStyleJSON</changelog>.
• Update the migration guide, API Docs, Markdown files - Readme, Developing, etc

Summary of changes

This PR:

1. adds loadStyleURI and loadStyleJSON to MapboxMap
2. moves Style from MapView to MapboxMap (since the above methods return Style in the completion closure)