👌 IMPROVE: Substitution extension

[chrisjsewell]

Allow any value type (including dict, list, datetime) and emit a myst.substitution warning for errors in resolving the substitution.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.23 🎉

Comparison is base (b043351) 90.00% compared to head (e639ce7) 90.23%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #777      +/-   ##
==========================================
+ Coverage   90.00%   90.23%   +0.23%     
==========================================
  Files          23       23              
  Lines        2970     2970              
==========================================
+ Hits         2673     2680       +7     
+ Misses        297      290       -7     

Flag	Coverage Δ
pytests	90.23% <100.00%> (+0.23%)	⬆️

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Impacted Files	Coverage Δ
myst_parser/config/dc_validators.py	83.63% <100.00%> (+0.30%)	⬆️
myst_parser/config/main.py	86.26% <100.00%> (ø)
myst_parser/mdit_to_docutils/base.py	93.41% <100.00%> (+0.72%)	⬆️
myst_parser/warnings_.py	96.22% <100.00%> (+0.07%)	⬆️

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[firasm]

Thank you, this is great!!