Expose myst_substitions to RST

[chancez]

While this doesn't fix the fact that .. replace:: does not work within eval-rst, it does make it possible to perform substitutions within eval-rst using the same substitutions available to Jinja substitutions.

Related: #680

Honestly I have very little experience with docutils, or even MyST but I basically figured out substitutions need to be registered by calling self.document.note_substitution_def from reading where the Undefined substitution referenced: error message comes from and determining that it reads from two dictionaries that the document.note_substitution_def updates. I then kind of got lucky and found that MyST already called the necessary function for the wordcount extension, which gave me a basic example of what I needed to do. I literally just copy and pasted the code used for the word count extension but using the values from self.md_config.substitutions which is where myst_substitutions values are stored, and it "just worked".

Let me know what you think, like I said, it was really just a stab at trying to make something substitution related work for eval-rst. I'd still love to get .. replace:: working but this will solve my needs pretty well I think.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.29%. Comparing base (1038800) to head (30e52eb).

Files	Patch %	Lines
myst_parser/mdit_to_docutils/base.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #966      +/-   ##
==========================================
- Coverage   90.30%   90.29%   -0.01%     
==========================================
  Files          24       24              
  Lines        3507     3514       +7     
==========================================
+ Hits         3167     3173       +6     
- Misses        340      341       +1     

Flag	Coverage Δ
pytests	90.29% <85.71%> (-0.01%)	⬇️

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