Fix bug in LngLatBounds.extend by allowing object literal LngLat as a parameter.

[stampyzfanz]

Launch Checklist

Fixes #12095

Changes

Checks if LngLatBounds.extend was passed an object literal LngLat (v3). This fixes a bug where it was treated like null and made no changes to the LngLatBounds object.

• include before/after visuals or gifs if this PR includes visual changes
• write tests for all new functionality
• document any changes to public APIs. No changes to public APIs
• post benchmark scores - I can't seem to set up the benchmark software locally. "'BENCHMARK_VERSION' is not recognized as an internal or external command". I can't imagine it has substantially worse performance based on the nature of the change.
• manually test the debug page
• tagged @mapbox/map-design-team @mapbox/static-apis if this PR includes style spec API or visual changes
• tagged @mapbox/gl-native if this PR includes shader changes or needs a native port
  I don't think it needs a native port.
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
  Can someone apply the bug label please I don't believe I have permission as a community member.
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Add object literal support in LngLatBounds.extend</changelog>

Sorry about all the commits: yarn gave me errors when trying to perform the unit tests locally so I did unit tests tests on GitHub in this PR. They'll be squashed when merged though.

[CLAassistant]

All committers have signed the CLA.

[stepankuzmin]

Looks good to me, thanks @stampyzfanz!