Change geojson source API to use promises

[HarelM]

Change the geojson source API from using callbacks to using promises.
Also changed the examples to use that and verified that they work.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 75.56%. Comparing base (10af384) to head (6e70163).
Report is 848 commits behind head on main.

Files with missing lines	Patch %	Lines
src/source/geojson_source.ts	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3421      +/-   ##
==========================================
+ Coverage   75.50%   75.56%   +0.05%     
==========================================
  Files         241      242       +1     
  Lines       19230    19201      -29     
  Branches     4271     4259      -12     
==========================================
- Hits        14520    14509      -11     
+ Misses       4710     4692      -18     

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