Treesitter parser (#2902)

A push towards better editor support.
See utils/treesitter/README.md

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Co-authored-by: josh11b <josh11b@users.noreply.github.com>

utils/treesitter/README.md

@@ -0,0 +1,27 @@
+<!--
+Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+Exceptions. See /LICENSE for license information.
+SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+-->
+
+# Tree-sitter grammar for Carbon
+
+Tree-sitter is currently used for syntax highlighting in supported editors.
+
+## Editor Installation
+
+### Helix
+
+1. Install
+   [tree-sitter](https://tree-sitter.github.io/tree-sitter/creating-parsers#installation)
+   and Nodejs.
+2. Install [Helix](https://docs.helix-editor.com/install.html).
+3. Run `./helix.sh`
+
+### Neovim
+
+TODO
+
+### Emacs
+
+TODO