👌 IMPROVE: MyST target block parsing

[chrisjsewell]

Match characters instead of using regex.
This now allows for an unlimited target length and whitespace in the target.
Also improve default rendering, inline with https://github.com/executablebooks/myst-vs-code

closes #28

[codecov]

Codecov Report

Merging #31 (1bd859a) into master (bb06119) will decrease coverage by 0.05%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master      #31      +/-   ##
==========================================
- Coverage   93.26%   93.20%   -0.06%     
==========================================
  Files          24       24              
  Lines        1233     1237       +4     
==========================================
+ Hits         1150     1153       +3     
- Misses         83       84       +1     

Flag	Coverage Δ
pytests	93.20% <90.90%> (-0.06%)	⬇️

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Impacted Files	Coverage Δ
mdit_py_plugins/myst_blocks/index.py	95.55% <90.90%> (-0.96%)	⬇️

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