✨ Allow for use of the line-block directive

[chrisjsewell]

The line-block directive constructs a block where all line breaks (and also initial indentations) are respected.

This was not previously supported, due to a missing State method

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 81.81818% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 90.07%. Comparing base (dfaf6d7) to head (bb79d80).

Files	Patch %	Lines
myst_parser/mocking.py	81.81%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #900      +/-   ##
==========================================
- Coverage   90.12%   90.07%   -0.06%     
==========================================
  Files          24       24              
  Lines        3383     3405      +22     
==========================================
+ Hits         3049     3067      +18     
- Misses        334      338       +4     

Flag	Coverage Δ
pytests	90.07% <81.81%> (-0.06%)	⬇️

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