👌 IMPROVE: Directive option parsing

[chrisjsewell]

Previously, directive options blocks have been parsed using a "full" YAML parser.
This is unnecessary, and problematic since only a mapping of string key to string value is required,
not any of the other YAML constructs; sequences, flow style, anchors, aliases, tags, ...,
It required addition handling of non-string values (failing or converting back to strings),
and could also cause confusion when values were not parsed as strings (see e.g. #712).

This commit, introduces a new "restricted" YAML parser (adapted from pyyaml),
which will maintain back-compatibilty with existing options blocks, but will only parse mappings of string keys to string values (including multi-line strings and comment parsing).
Parsing errors are also improved, by reporting the exact line on which the error occured.

closes #382

[codecov]

Codecov Report

Attention: 46 lines in your changes are missing coverage. Please review.

Comparison is base (1f61fa0) 90.26% compared to head (86dc085) 90.08%.

Files	Patch %	Lines
myst_parser/parsers/options.py	89.57%	39 Missing ⚠️
myst_parser/parsers/directives.py	88.67%	6 Missing ⚠️
myst_parser/mocking.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #796      +/-   ##
==========================================
- Coverage   90.26%   90.08%   -0.19%     
==========================================
  Files          23       24       +1     
  Lines        2970     3360     +390     
==========================================
+ Hits         2681     3027     +346     
- Misses        289      333      +44     

Flag	Coverage Δ
pytests	90.08% <89.27%> (-0.19%)	⬇️

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