refactor: Use typed interfaces for generating EnvFilters from config

[RasmusRendal]

I think this is just nicer than returning a string that the consumer is
expected to parse.

I wish we could do this without any string parsing at all, but this
functionality is still missing.

This is arguably not a super important PR, but it's just something weird I spotted and wanted to fix.

[codecov]

Codecov Report

❌ Patch coverage is 92.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.91%. Comparing base (eac2573) to head (c1d109a).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/config.rs	95.23%	1 Missing ⚠️
src/lib.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
+ Coverage   74.18%   74.91%   +0.72%     
==========================================
  Files           5        5              
  Lines         275      283       +8     
==========================================
+ Hits          204      212       +8     
  Misses         71       71              

Files with missing lines	Coverage Δ
src/config.rs	70.76% <95.23%> (+4.10%)	⬆️
src/lib.rs	88.29% <75.00%> (ø)

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