Run pyodide conversion in subprocess

[philippjfr]

Allows for parallel conversion and isolates the environment the conversion happens in.

[codecov]

Codecov Report

Merging #3878 (48c4cfb) into master (c8ae3d9) will decrease coverage by 0.00%.
The diff coverage is 9.09%.

@@            Coverage Diff             @@
##           master    #3878      +/-   ##
==========================================
- Coverage   82.85%   82.84%   -0.01%     
==========================================
  Files         215      215              
  Lines       32381    32372       -9     
==========================================
- Hits        26829    26820       -9     
  Misses       5552     5552              

Flag	Coverage Δ
ui-tests	34.38% <9.09%> (-0.01%)	⬇️
unitexamples-tests	74.92% <9.09%> (-0.03%)	⬇️

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Impacted Files	Coverage Δ
panel/command/convert.py	64.28% <ø> (ø)
panel/io/convert.py	29.57% <9.09%> (-0.87%)	⬇️
panel/io/reload.py	69.56% <0.00%> (-2.18%)	⬇️
panel/tests/command/test_serve.py	83.54% <0.00%> (-1.46%)	⬇️
panel/io/state.py	69.16% <0.00%> (-0.19%)	⬇️
panel/tests/util.py	89.21% <0.00%> (-0.11%)	⬇️
panel/io/jupyter_server_extension.py	0.00% <0.00%> (ø)
panel/tests/ui/widgets/test_tabulator.py	99.06% <0.00%> (+0.10%)	⬆️
panel/pane/idom.py	28.36% <0.00%> (+0.19%)	⬆️
panel/tests/conftest.py	88.59% <0.00%> (+0.38%)	⬆️
... and 1 more

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