Ensure _bundle_css is added as resource and resolved on baseclass

[philippjfr]

This PR introduces two changes to allow resolving the new _bundle_css for ESM components.

Specifically we:

• Ensure that the bundle CSS is resolved on the baseclass
• The _bundle_css files are added to the document head.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 2 lines in your changes missing coverage. Please review.

Project coverage is 86.77%. Comparing base (8463ff8) to head (d70c840).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/custom.py	85.71%	1 Missing ⚠️
panel/io/resources.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7691      +/-   ##
==========================================
+ Coverage   86.27%   86.77%   +0.49%     
==========================================
  Files         346      346              
  Lines       52464    52473       +9     
==========================================
+ Hits        45263    45533     +270     
+ Misses       7201     6940     -261     

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