feat: add syntax highlighting for 9 additional languages

- Cargo.toml: Add tree-sitter grammars for Nix, Python, JavaScript,
  TypeScript, Go, C, CSS, HTML, YAML. Upgrade tree-sitter-highlight
  to 0.26 for language version 15 compatibility.

- src/highlight.rs: Add Language enum variants and get_config()
  match arms for all new languages. Update render() callback for
  0.26 API (writes attributes to buffer). Add tests for Nix and
  Python highlighting.

TOML excluded due to incompatible API (tree-sitter 0.20 vs 0.26).

Cargo.toml

@@ -13,10 +13,19 @@ thiserror      = "2"
 walkdir        = "2"
 
 # Syntax highlighting
-tree-sitter-bash      = "0.23"
-tree-sitter-highlight = "0.24"
-tree-sitter-json      = "0.24"
-tree-sitter-rust      = "0.23"
+tree-sitter-bash       = "0.23"
+tree-sitter-c          = "0.24"
+tree-sitter-css        = "0.25"
+tree-sitter-go         = "0.25"
+tree-sitter-highlight  = "0.26"
+tree-sitter-html       = "0.23"
+tree-sitter-javascript = "0.25"
+tree-sitter-json       = "0.24"
+tree-sitter-nix        = "0.3"
+tree-sitter-python     = "0.25"
+tree-sitter-rust       = "0.23"
+tree-sitter-typescript = "0.23"
+tree-sitter-yaml       = "0.7"
 
 # CSS processing
 lightningcss = "1.0.0-alpha.70"