✨ NEW: Add fieldlist extension

[chrisjsewell]

Field lists are mappings from field names to field bodies,
based on the reStructureText syntax.

A prominent use case of field lists is for use in API docstrings, as used in Sphinx's docstring renderers.
This should hopefully pave the way for use with sphinx.ext.autodoc

[codecov]

Codecov Report

Merging #455 (b9730e0) into master (cf18116) will decrease coverage by 0.01%.
The diff coverage is 90.32%.

@@            Coverage Diff             @@
##           master     #455      +/-   ##
==========================================
- Coverage   90.86%   90.84%   -0.02%     
==========================================
  Files          16       16              
  Lines        1882     1912      +30     
==========================================
+ Hits         1710     1737      +27     
- Misses        172      175       +3     

Flag	Coverage Δ
pytests	90.84% <90.32%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
myst_parser/docutils_renderer.py	92.80% <88.46%> (-0.19%)	⬇️
myst_parser/main.py	87.80% <100.00%> (+0.30%)	⬆️
myst_parser/mocking.py	86.64% <100.00%> (+0.05%)	⬆️

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