Allow ESM components to load CSS bundles

[philippjfr]

Currently we support the ability to compile a JS bundle to be loaded by JS components. However sometimes you want to bundle additional resources, such as a separate CSS bundle and any associated assets (e.g. images, fonts etc.). This PR makes it possible to:

• Load a CSS bundle that sits alongside the JS bundle
• Tell esbuild to load and bundle additional assets using the --file-loaders argument

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 86.76%. Comparing base (101d6d2) to head (8a22a18).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/custom.py	82.60%	4 Missing ⚠️
panel/command/compile.py	0.00%	3 Missing ⚠️
panel/io/compile.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #7685       +/-   ##
===========================================
+ Coverage   71.65%   86.76%   +15.11%     
===========================================
  Files         345      346        +1     
  Lines       52443    52464       +21     
===========================================
+ Hits        37577    45522     +7945     
+ Misses      14866     6942     -7924     

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