👌 Improve: Non-directive colon-fences

[chrisjsewell]

At present, colon fences without a directive specified (e.g. :::{note}) are treated as code bocks

:::name
Some text
:::

<code language=name>
    Some text

In keeping with the purpose of these block, as containers for nested MyST content, they are now converted to:

<div class=name>
   <paragraph>
      Some text

This is also in keeping with the original pandoc/djot inspiration https://htmlpreview.github.io/?https://github.com/jgm/djot/blob/master/doc/syntax.html#div

[chrisjsewell]

cc @rowanc1 @choldgraf

[rowanc1]

Makes sense to me, and like that there is a difference between colons and backticks.

[codecov]

Codecov Report

Base: 89.31% // Head: 89.36% // Increases project coverage by +0.05% 🎉

Coverage data is based on head (599202e) compared to base (ac111ea).
Patch coverage: 83.33% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #713      +/-   ##
==========================================
+ Coverage   89.31%   89.36%   +0.05%     
==========================================
  Files          23       23              
  Lines        2844     2867      +23     
==========================================
+ Hits         2540     2562      +22     
- Misses        304      305       +1     

Flag	Coverage Δ
pytests	89.36% <83.33%> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
myst_parser/mdit_to_docutils/base.py	92.88% <75.00%> (+0.07%)	⬆️
myst_parser/parsers/docutils_.py	80.81% <91.66%> (+0.81%)	⬆️
myst_parser/sphinx_ext/main.py	89.74% <100.00%> (+0.26%)	⬆️

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