feature: "==" text highlighting.

[Dani-v-tue]

Did you check the tree-sitter docs?

• I have read all the tree-sitter docs if it relates to using the parser

Is your feature request related to a problem? Please describe.

Many widely used markdown editors support using ==highlight text==. It would be nice if this was supported directly through tree-sitter-markdown

Describe the solution you'd like

Add an extension highlight group that captures ==text between double equal signs==.

Describe alternatives you've considered

It is possible to manually add it as a highlight group.

For my specific case obsidian.nvim has a feature for this, except it is buggy and the whole plugin is quite bloated.

Additional context

If people think this has merit I could attempt to create a PR for this, although I am quite inexperienced in writing tree-sitter grammars.