Unify icon contributions and clarify supported formats

[brisvag]

This PR aims to unify a bit the manifest regarding icons, and to bring it more in line with what the underlying app-model accepts.

We actually "officially" support more things than appmodel (e.g: https urls), and I'm not sure whether we plan to keep these and handle the extra logic ourselves or what else to do. At least with this we start supporting dark/light for all icon fields.

[willingc]

Looks good. Seems to have a circular import that in the tests.

[brisvag]

Since it's not reused, I just moved the validator to the model where it's needed.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.89%. Comparing base (26804e3) to head (af10fdb).

Files with missing lines	Patch %	Lines
src/npe2/manifest/schema.py	80.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #450      +/-   ##
===========================================
- Coverage   100.00%   99.89%   -0.11%     
===========================================
  Files           37       37              
  Lines         2780     2785       +5     
===========================================
+ Hits          2780     2782       +2     
- Misses           0        3       +3     

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