Split PanelJupyterExecutor into separate module

[philippjfr]

Fixes #4271

[codecov]

Codecov Report

Merging #4276 (80adfd1) into main (7918671) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #4276      +/-   ##
==========================================
- Coverage   82.72%   82.69%   -0.03%     
==========================================
  Files         238      239       +1     
  Lines       34382    34393      +11     
==========================================
  Hits        28442    28442              
- Misses       5940     5951      +11     

Flag	Coverage Δ
ui-tests	38.01% <0.00%> (-0.02%)	⬇️
unitexamples-tests	73.83% <0.00%> (-0.03%)	⬇️

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Impacted Files	Coverage Δ
panel/io/jupyter_executor.py	0.00% <0.00%> (ø)
panel/io/jupyter_server_extension.py	0.00% <0.00%> (ø)

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