Add ability to add global loading spinner to application(s)

[philippjfr]

In converted pyodide applications we have had the ability to add a global loading indicator for a long time. There is no reason not to also support it for other applications that may take a little while to load.

[codecov]

Codecov Report

Merging #4659 (c4a8340) into main (ce4d940) will decrease coverage by 12.60%.
The diff coverage is 69.56%.

@@             Coverage Diff             @@
##             main    #4659       +/-   ##
===========================================
- Coverage   83.19%   70.60%   -12.60%     
===========================================
  Files         264      266        +2     
  Lines       37351    37415       +64     
===========================================
- Hits        31076    26416     -4660     
- Misses       6275    10999     +4724     

Flag	Coverage Δ
ui-tests	?
unitexamples-tests	70.60% <69.56%> (-2.67%)	⬇️

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Impacted Files	Coverage Δ
panel/command/serve.py	38.46% <ø> (+2.51%)	⬆️
panel/io/pyodide.py	0.00% <0.00%> (ø)
panel/io/resources.py	84.04% <33.33%> (-0.41%)	⬇️
panel/tests/ui/io/test_loading.py	61.11% <61.11%> (ø)
panel/io/server.py	76.66% <85.71%> (+0.02%)	⬆️
panel/io/document.py	61.53% <87.50%> (+8.39%)	⬆️
panel/config.py	68.19% <100.00%> (+0.07%)	⬆️

... and 68 files with indirect coverage changes

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