Update dollarmath to be inline with JS version

[chrisjsewell]

I've just created markdown-it-dollarmath, and in particular here: executablebooks/markdown-it-dollarmath@e3eb244, I added the extra token math_inline_double and renamed math_block_eqno to math_block_label.
Also, there is the labelNormalizer option and the render hook (and similarly with amsmath).

Note, this will also require a change in myst-parser, so should be followed by a minor (breaking) version release

Should link to the JS plugin demonstration pages as well.

[chrisjsewell]

should probably also make double inline on by default as well (and in myst-parser)