gfm reader/writer?: support new inline and display math formats

[jgm]

GitHub now allows this alternative syntax for inline math:

$`x=y^2`$

This should be supported by the reader (will require support in jgm/commonmark-hs) and perhaps the writer.

Also for display math:

``` math
x = y^2 +
3
```

[jgm]

Ref: https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/writing-mathematical-expressions

[jgm]

$x=y$

[jgm]

issue9121 branch:

• Add extension Ext_tex_math_gfm.
• Add this to gfm extensions (in addition to of Ext_tex_math_dollars)
• Document in MANUAL.txt. tex_math_gfm adds the two new verbatim constructs.
• Implement the extension as a pass over the AST after parsing in Text.Pandoc.Readers.Commonmark. Transform: InlineMath that starts and ends with equal-length backtick spans to InlineMath with these stripped off; CodeBlock with class math to Math display. [or should this go in commonmark-extensions?]

[jgm]

$$ x = 2 $$

[jgm]

$x = \text{y and $z$}$

$x = \text{y and $z$}$

[digitalmoksha]

I think this support is awesome! 🚀

Just have to say that GitLab pioneered/added (ok I don't know if it was the absolute first one) in 2016, https://about.gitlab.com/releases/2016/12/22/gitlab-8-15-released/, in their GLFM syntax. 😄