Support collapsed cells

[mwouts]

Will close https://github.com/mwouts/jupytext/issues/1040

[codecov[bot]]

Codecov Report

Merging #1042 (6b35e60) into main (e097422) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1042      +/-   ##
==========================================
+ Coverage   99.02%   99.04%   +0.01%     
==========================================
  Files         119      119              
  Lines       11030    11088      +58     
==========================================
+ Hits        10923    10982      +59     
+ Misses        107      106       -1     

Impacted Files	Coverage Δ
jupytext/cell_metadata.py	98.99% <100.00%> (+0.01%)	:arrow_up:
jupytext/cell_to_text.py	97.83% <100.00%> (ø)
jupytext/header.py	98.78% <100.00%> (ø)
jupytext/jupytext.py	98.44% <100.00%> (+0.04%)	:arrow_up:
jupytext/metadata_filter.py	100.00% <100.00%> (ø)
jupytext/version.py	100.00% <100.00%> (ø)
tests/conftest.py	100.00% <100.00%> (ø)
tests/test_cell_metadata.py	100.00% <100.00%> (ø)
tests/utils.py	100.00% <100.00%> (ø)
jupytext/cell_reader.py	96.79% <0.00%> (+0.16%)	:arrow_up:

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