Add support for Sanity v3 (code ready)

I have a working version of this plugin for Sanity v3. @Grsmto would you be interested in a PR?

It is basically a fork of `@sanity/google-maps-input` that swaps the Google Maps bits for Mapbox (I believe this is what you did when you created the plugin after looking at the code for guidance).

It still uses the old `mapbox-gl@v1` so users from your plugin (like myself) don't need to consider any of the policy or pricing changes that came with v2. In case someone needs this to be updated we could look into it, I guess.

Let me know what you think and if this is something that could be of use to anyone 👍

Btw, your plugin still works fine for me with v0.0.3 so perhaps if v0.0.4 is not working we could also look into what regression was introduced and ship a fix for users with sanity v2?

Start by reviewing the existing plugin and comparing it with @sanity/google-maps-input to understand the proposed Sanity v3 fork. Check the reported difference between v0.0.3 and v0.0.4, and confirm how the proposed mapbox-gl@v1 implementation should support Sanity v3. Done means the maintainers agree on the scope and the v3 support path, with any Sanity v2 regression handled separately.

Written by the indexing model from the issue text.