👌 Improve default slug generation for heading anchors

[Cimbali]

This can be easily tested by visiting the source markdown page on github:

https://github.com/Cimbali/MyST-Parser/blob/master/tests/test_sphinx/sourcedirs/references/index.md#image-in-title-

Fixes #752.

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Note that for the nested $a=1$ case I’ve updated the test to match the new behaviour, to allow improving matching github behaviour bit by bit.

Neither the old or new test match Github behaviour:
https://github.com/executablebooks/MyST-Parser/blob/master/tests/test_sphinx/sourcedirs/references/index.md#title-with-nested-a1

The issue seems to be math-to-text conversion (so the trailing - added by this PR is “more correct” even though not 100% there).

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[chrisjsewell]

Thanks @Cimbali! I'll have a proper look probably over the weekend

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.03 🎉

Comparison is base (80b892f) 90.23% compared to head (f5c7809) 90.26%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #753      +/-   ##
==========================================
+ Coverage   90.23%   90.26%   +0.03%     
==========================================
  Files          23       23              
  Lines        2970     2970              
==========================================
+ Hits         2680     2681       +1     
+ Misses        290      289       -1     

Flag	Coverage Δ
pytests	90.26% <100.00%> (+0.03%)	⬆️

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	93.52% <100.00%> (+0.10%)	⬆️

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[Cimbali]

Hi @chrisjsewell have you been able to have a look?

It’s quite simply keeping the trailing - in slugs, as you can see for example on the first page I linked.

[chrisjsewell]

Ok cool I've checked this and seems good thanks 👌

My main concern was that removing the strip would break the behaviour for other situations.
But I've confirmed that the Markdown parsing of headings already:

1. Only starts parsing the title after any initial whitespace
2. Strips any text whitespace that occurs directly before a newline

So e.g. # title will create a text token with content "title" (not " title "), such that whitespace is already stripped, and you won't get a slug like -title-
(See the markdown-it online tool)

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