Add YAML/TOML frontmatter injections for markdown (#21503)

Closes #7938. Adds front-matter injections for TOML/YAML in markdown. 
- See: https://github.com/tree-sitter-grammars/tree-sitter-markdown/blob/split_parser/tree-sitter-markdown/queries/injections.scm.

Co-authored-by: Peter Tripp <peter@zed.dev>

crates/languages/src/markdown/injections.scm

@@ -8,3 +8,7 @@
 
 ((html_block) @content
   (#set! "language" "html"))
+
+((minus_metadata) @content (#set! "language" "yaml"))
+
+((plus_metadata) @content (#set! "language" "toml"))