Allow for Vega selection fields

[philippjfr]

• Fixes Failed to parse Vega-lite selection using Altair #5133
• Adds tests

[codecov]

Codecov Report

Merging #5145 (f63a4f7) into main (3c41978) will decrease coverage by 0.02%.
The diff coverage is 85.13%.

@@            Coverage Diff             @@
##             main    #5145      +/-   ##
==========================================
- Coverage   83.73%   83.71%   -0.02%     
==========================================
  Files         274      274              
  Lines       39414    39488      +74     
==========================================
+ Hits        33003    33059      +56     
- Misses       6411     6429      +18     

Flag	Coverage Δ
ui-tests	40.52% <82.43%> (+0.10%)	⬆️
unitexamples-tests	73.68% <20.27%> (-0.13%)	⬇️

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Impacted Files	Coverage Δ
panel/pane/vega.py	78.78% <10.00%> (-3.13%)	⬇️
panel/tests/ui/pane/test_vega.py	97.43% <96.87%> (-2.57%)	⬇️

... and 5 files with indirect coverage changes

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