Add `area` endpoint support

[mpiannucci]

• Add area endpoint
• Move selection logic to its own module
• add tests for selection

[mpiannucci]

pre-commit.ci autofix

[mpiannucci]

pre-commit.ci autofix

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 74.22680% with 25 lines in your changes missing coverage. Please review.

Project coverage is 81.64%. Comparing base (434d6b9) to head (83458f3). Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
xpublish_edr/select.py	72.30%	18 Missing :warning:
xpublish_edr/plugin.py	77.41%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #49 +/- ## ========================================== + Coverage 79.02% 81.64% +2.62% ========================================== Files 6 7 +1 Lines 205 267 +62 ========================================== + Hits 162 218 +56 - Misses 43 49 +6 ```

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[mpiannucci]

Love when code coverage goes down from adding more tests lol

[mpiannucci]

Agree with this feedback, especially because I want to add multipoint support next which further complicates things

I'll do some cleaning before merging :)

[mpiannucci]

@abkfenris Let me know how you feel about this reorg

[abkfenris]

@mpiannucci Much easier to follow, thank you.