👌 IMPROVE: Code block highlighting

[chrisjsewell]

In markdown a code block (a.k.a fence) is of the form:

```language
source text
```

MyST-Parser mimics the code-block directive to render these blocks:

In sphinx, the lexer name is only recorded as the language attribute,
and the text is lexed later by pygments within the visit_literal_block
method of the output format SphinxTranslator.
This is the current logic.

However, in docutils, this directive directly parses the text with the pygments lexer, if syntax highlighting is enabled (the default). This was not handled.

Both cases are now handled, and additionally the following configuration are added:

• myst_highlight_code_blocks (docutils only): If True (default) use pygments to create lexical tokens for the given language, otherwise skip lexical analysis
• myst_number_code_blocks: A list of languages to add line numbers to

[codecov]

Codecov Report

Merging #478 (3620554) into master (6c44075) will decrease coverage by 0.16%.
The diff coverage is 85.36%.

@@            Coverage Diff             @@
##           master     #478      +/-   ##
==========================================
- Coverage   90.19%   90.02%   -0.17%     
==========================================
  Files          16       16              
  Lines        2030     2056      +26     
==========================================
+ Hits         1831     1851      +20     
- Misses        199      205       +6     

Flag	Coverage Δ
pytests	90.02% <85.36%> (-0.17%)	⬇️

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Impacted Files	Coverage Δ
myst_parser/docutils_renderer.py	92.13% <80.64%> (-0.53%)	⬇️
myst_parser/__init__.py	88.57% <100.00%> (+0.33%)	⬆️
myst_parser/main.py	88.80% <100.00%> (-0.28%)	⬇️

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