DensMAP support

[keller-mark]

I tried to follow the feedback described in a previous PR that contributed DensMAP https://github.com/scverse/scanpy/pull/2684#issuecomment-1764564449 but re-implemented on top of the state of the current scanpy main branch.

I did not add release notes because the contributor guide says to wait for PR feedback https://scanpy.readthedocs.io/en/latest/dev/documentation.html#adding-to-the-docs

• [x] Closes #1619
• [x] Closes #2684
• [x] Tests included or not required because:

• [x] Release notes not necessary because:

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 76.66%. Comparing base (c6766d7) to head (603afb3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2946      +/-   ##
==========================================
+ Coverage   76.63%   76.66%   +0.03%     
==========================================
  Files         109      109              
  Lines       12533    12545      +12     
==========================================
+ Hits         9605     9618      +13     
+ Misses       2928     2927       -1     

Files	Coverage Δ
src/scanpy/tools/__init__.py	91.30% <100.00%> (ø)
src/scanpy/tools/_umap.py	77.77% <100.00%> (+6.11%)	:arrow_up: