Handle config.raw_css and config.css_files compatibility with shadow DOM

[philippjfr]

Provides initial layer of backward compatibility for config.raw_css and config.css_files by stripping out any .bk prefixes and adding the contents of these variables to the shadow DOM of ALL components. We have to separately decide a policy around either deprecating or messaging to the user how expensive injecting CSS into every single component is.

[codecov]

Codecov Report

Merging #4508 (aa7dd21) into main (c4b7695) will decrease coverage by 11.16%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##             main    #4508       +/-   ##
===========================================
- Coverage   82.14%   70.99%   -11.16%     
===========================================
  Files         243      243               
  Lines       35556    35569       +13     
===========================================
- Hits        29209    25253     -3956     
- Misses       6347    10316     +3969     

Flag	Coverage Δ
ui-tests	?
unitexamples-tests	70.99% <100.00%> (-3.06%)	⬇️

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Impacted Files	Coverage Δ
panel/io/resources.py	82.00% <100.00%> (+0.20%)	⬆️
panel/reactive.py	79.61% <100.00%> (-0.73%)	⬇️
panel/pane/vtk/synchronizable_deserializer.py	0.00% <0.00%> (-97.59%)	⬇️
panel/tests/ui/widgets/test_tabulator.py	9.45% <0.00%> (-88.78%)	⬇️
panel/tests/ui/widgets/test_input.py	11.34% <0.00%> (-88.13%)	⬇️
panel/tests/ui/layout/test_accordion.py	14.72% <0.00%> (-85.28%)	⬇️
panel/tests/ui/widgets/test_sliders.py	15.42% <0.00%> (-84.58%)	⬇️
panel/tests/ui/layout/test_card.py	17.79% <0.00%> (-82.21%)	⬇️
panel/pane/vtk/synchronizable_serializer.py	1.11% <0.00%> (-76.56%)	⬇️
panel/tests/ui/pane/test_image.py	25.80% <0.00%> (-74.20%)	⬇️
... and 45 more

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