Improve ReactiveHTML loop support and validation

[philippjfr]

Fixes #3802

[codecov]

Codecov Report

Merging #3813 (50df3c8) into master (a4ff6ed) will increase coverage by 0.03%.
The diff coverage is 97.26%.

@@            Coverage Diff             @@
##           master    #3813      +/-   ##
==========================================
+ Coverage   82.97%   83.01%   +0.03%     
==========================================
  Files         221      221              
  Lines       32230    32285      +55     
==========================================
+ Hits        26744    26802      +58     
+ Misses       5486     5483       -3     

Flag	Coverage Δ
ui-tests	34.13% <34.24%> (-0.01%)	⬇️
unitexamples-tests	75.50% <97.26%> (+0.05%)	⬆️

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Impacted Files	Coverage Δ
panel/models/reactive_html.py	82.84% <92.85%> (+2.28%)	⬆️
panel/reactive.py	78.34% <100.00%> (+0.02%)	⬆️
panel/tests/test_reactive.py	99.07% <100.00%> (+0.14%)	⬆️
panel/io/reload.py	69.56% <0.00%> (-2.18%)	⬇️
panel/tests/command/test_serve.py	83.54% <0.00%> (-1.46%)	⬇️
panel/io/cache.py	81.98% <0.00%> (-0.46%)	⬇️
panel/io/state.py	70.66% <0.00%> (-0.22%)	⬇️
panel/io/server.py	74.85% <0.00%> (-0.20%)	⬇️
panel/io/jupyter_server_extension.py	0.00% <0.00%> (ø)
panel/tests/ui/widgets/test_tabulator.py	98.97% <0.00%> (+0.11%)	⬆️
... and 4 more

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