Add MDX language support

[nicodoggie]

Summary

Adds MDX as a supported language so path extraction works for .mdx files.

Changes

• Registers MDX in the Zed extension language list.
• Adds mdx to the internal Language enum.
• Routes MDX documents through the existing Markdown tree-sitter parser and extraction path.
• Adds a regression test covering MDX imports, Markdown image links, and JSX-style path attributes.

Validation

• cargo test test_mdx_extract_strings_with_markdown_parser

[kunlinglio]

Thanks for your contribution. Looks good to me!

And the tree_sitter_supported assertion is a nice touch too, I should add that to my other tests :)

I'll add a changelog entry and merge it in.

[kunlinglio]

LGTM