Add subs support for inline code snippets

[theletterf]

Fixes #1308

This adds the following notation for inline code subs:

{subs}`This is an inline snippet with {{version.stack}} a sub`

[github-actions]

🔍 Preview links for changed docs

• docs/syntax/substitutions.md

[Mpdreamz]

This this is technically a myst role syntax ensure we use our RoleParser<>

https://myst-parser.readthedocs.io/en/latest/syntax/roles-and-directives.html#roles-an-in-line-extension-point

Given its syntax role{args} we should name to role more as a noun/verb.

sub{args} ?

[theletterf]

@Mpdreamz Set this as a {subs} role, WDYT?

[Mpdreamz]

Love it!