Remove Dask as a hard dependency of Datashader

[hoxbro]

Resolves #1349

[jbednar]

Wow, when we discussed this on Discord yesterday I thought this would go onto the backlog and never get done, but here it is already being addressed! What a nice feeling! Looks good so far; just what I'd expect to see.

[codecov]

Codecov Report

Attention: Patch coverage is 63.28125% with 47 lines in your changes missing coverage. Please review.

Project coverage is 88.39%. Comparing base (083b93b) to head (dbed0b1).
Report is 1 commits behind head on main.

Files	Patch %	Lines
datashader/bundling.py	36.36%	7 Missing ⚠️
datashader/tests/test_raster.py	68.42%	6 Missing ⚠️
datashader/tiles.py	33.33%	6 Missing ⚠️
datashader/tests/test_transfer_functions.py	55.55%	4 Missing ⚠️
datashader/tests/test_xarray.py	60.00%	4 Missing ⚠️
datashader/core.py	75.00%	3 Missing ⚠️
datashader/resampling.py	66.66%	3 Missing ⚠️
datashader/tests/test_quadmesh.py	76.92%	3 Missing ⚠️
datashader/tests/test_dask.py	66.66%	2 Missing ⚠️
datashader/tests/test_geopandas.py	50.00%	2 Missing ⚠️
... and 4 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1350      +/-   ##
==========================================
- Coverage   88.57%   88.39%   -0.18%     
==========================================
  Files          93       93              
  Lines       18642    18722      +80     
==========================================
+ Hits        16512    16550      +38     
- Misses       2130     2172      +42     

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[hoxbro]

I can see we need to update HoloViews at least in one place:

https://github.com/holoviz/holoviews/blob/eb79ab52414f53b5990f2b624edd4ba582a3fa6e/holoviews/operation/datashader.py#L5