rst Support

[brian6932]

Is your feature request related to a problem? Please describe.
ReStructured Text's kinda like Markdown, and with this GH issue being a thing https://github.com/orgs/community/discussions/86715. It's a better time than any to add support.

Describe the solution you'd like
An rst mode, prob with a flag and file extension check, shouldn't be default ofc.

Describe alternatives you've considered
Opening rst files in glow currently renders them as Markdown, which's kinda close, but doesn't entirely work.