Git pathspecs and submodule recursion for `iter_gitworkstree()` with pathspec support

[mih]

This is yet another attempt towards pathspecs. It replaces #714 with something even more low-level and lightweight.

• Closes: #587

TODO

• [x] Clarify None vs [] for GitPathSpecs

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.75281% with 8 lines in your changes missing coverage. Please review.

Project coverage is 92.52%. Comparing base (c77087b) to head (1722e1f). Report is 99 commits behind head on main.

Files with missing lines	Patch %	Lines
datalad_next/gitpathspec/pathspec.py	98.31%	0 Missing and 2 partials :warning:
datalad_next/gitpathspec/tests/test_gitpathspec.py	97.29%	1 Missing and 1 partial :warning:
datalad_next/iter_collections/gitworktree.py	97.01%	2 Missing :warning:
...ext/iter_collections/tests/test_itergitworktree.py	95.12%	0 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #719 +/- ## ========================================== + Coverage 92.43% 92.52% +0.09% ========================================== Files 193 197 +4 Lines 14135 14440 +305 Branches 2134 2209 +75 ========================================== + Hits 13065 13360 +295 - Misses 806 810 +4 - Partials 264 270 +6 ```

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