Add YAML/TOML frontmatter injections for markdown

[uncenter]

Closes #7938. Adds the frontmatter injections, as done in https://github.com/tree-sitter-grammars/tree-sitter-markdown/blob/split_parser/tree-sitter-markdown/queries/injections.scm.

Before	After

Release Notes:

• Added YAML and TOML frontmatter highlighting for markdown

[notpeter]

--- is YAML frontmatter
+++ is TOML.
At least that's the convention used by hugo front-matter and Zola front-matter.

I am not aware of any system which assumes YAML for +++.

[uncenter]

Thanks for updating that, that was my experience as well but was just going with upstream. This lgtm.

[notpeter]

Thanks for this!