feat(highlight): add TOML syntax highlighting

Use tree-sitter-toml-ng v0.7.0 from tree-sitter-grammars, which is
compatible with tree-sitter 0.26 (unlike the older tree-sitter-toml).

- Add tree-sitter-toml-ng dependency
- Add Toml variant to Language enum
- Add TOML_CONFIG with crate's HIGHLIGHTS_QUERY

Cargo.toml

@@ -39,6 +39,7 @@ toml     = "0.8"
 # Diagram rendering
 mermaid-rs-renderer = { version = "0.1", default-features = false }
 tree-sitter-md      = "0.5.2"
+tree-sitter-toml-ng = "0.7.0"
 
 # Patch dagre_rust to fix unwrap on None bug
 [patch.crates-io]