feat: new holoviz plotting

[flying-sheep]

TODO

• dotplot
• more plots?
• export helpers like dot_size
• other backends demo
• selenium for export?
• example notebook(s)
• re-export A/AdDim (A = sc.pl.hv_init())

---

• Closes Holoviz based plotting #2656
• Tests included or not required because:

• Release notes not necessary because:

[codecov]

Codecov Report

❌ Patch coverage is 45.19231% with 228 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.57%. Comparing base (90311d3) to head (b2602c9).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/scanpy/plotting/_v2/_core.py	0.00%	128 Missing ⚠️
src/scanpy/plotting/_v2/_tl.py	0.00%	44 Missing ⚠️
src/scanpy/plotting/_v2/_api.py	0.00%	17 Missing ⚠️
src/scanpy/plotting/_v2/_pp.py	0.00%	17 Missing ⚠️
src/scanpy/plotting/legacy/mpl_settings.py	89.23%	7 Missing ⚠️
src/scanpy/plotting/_v2/__init__.py	0.00%	6 Missing ⚠️
src/scanpy/datasets/_datasets.py	57.14%	3 Missing ⚠️
src/scanpy/plotting/__init__.py	90.00%	2 Missing ⚠️
src/scanpy/tools/_embedding_density.py	60.00%	2 Missing ⚠️
src/scanpy/plotting/legacy/_utils.py	87.50%	1 Missing ⚠️
... and 1 more

Additional details and impacted files

@@                Coverage Diff                @@
##           aggregate-dim    #4188      +/-   ##
=================================================
- Coverage          80.02%   78.57%   -1.46%     
=================================================
  Files                121      128       +7     
  Lines              13012    13263     +251     
=================================================
+ Hits               10413    10421       +8     
- Misses              2599     2842     +243     

Flag	Coverage Δ
hatch-test.low-vers	77.43% <45.19%> (-1.48%)	⬇️
hatch-test.pre	78.47% <45.19%> (-1.47%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/scanpy/__init__.py	96.55% <100.00%> (ø)
src/scanpy/_settings/__init__.py	96.11% <ø> (+4.38%)	⬆️
src/scanpy/external/pl.py	29.21% <100.00%> (ø)
src/scanpy/plotting/_common.py	100.00% <100.00%> (ø)
src/scanpy/plotting/legacy/__init__.py	100.00% <100.00%> (ø)
src/scanpy/plotting/legacy/_anndata.py	85.65% <100.00%> (ø)
src/scanpy/plotting/legacy/_baseplot_class.py	80.36% <100.00%> (ø)
src/scanpy/plotting/legacy/_docs.py	100.00% <100.00%> (ø)
src/scanpy/plotting/legacy/_dotplot.py	91.89% <100.00%> (ø)
src/scanpy/plotting/legacy/_easter_egg.py	100.00% <ø> (ø)
... and 21 more

... and 2 files with indirect coverage changes