refactor: restructure lib and add support for `modular`, fixes #219

[MichaelSolati]

[pdandradeb]

hey @MichaelSolati, I see that all tests are passing. Is anything missing? Maybe I could help it.

[codecov-commenter]

Codecov Report

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@@           Coverage Diff           @@
##             main     #230   +/-   ##
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  Coverage        ?   84.18%           
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  Files           ?       16           
  Lines           ?      392           
  Branches        ?      100           
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  Hits            ?      330           
  Misses          ?       40           
  Partials        ?       22           

Flag	Coverage Δ
geofirestore	99.45% <0.00%> (?)
geofirestore-core	70.53% <0.00%> (?)

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