Set up pre-commit (#73)

Added here:

- .pre-commit-config.yaml
- docs/project/contribution_tools.md
- src/scripts/pre-commit-toc.js
- CONTRIBUTING.md
- proposals/* (just to switch TOC formats)

The rest consists of auto-generated fixes.

The switch from doctoc to markdown-toc is because doctoc wants to put a TOC in every file, whereas markdown-toc only does it if you have an insertion point.

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

src/scripts/pre-commit-toc.js

@@ -0,0 +1,28 @@
+#!/usr/local/bin/node
+
+/*
+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
+*/
+
+/*
+Runs markdown-toc for pre-commits. This is necessary to handle multiple file
+arguments, which markdown-toc does not.
+
+Humans should generally use markdown-toc following the install instructions
+from https://github.com/jonschlinkert/markdown-toc.
+*/
+
+const mdtoc = require('markdown-toc');
+const fs = require('fs');
+
+const files = process.argv.slice(2);
+for (var i = 0; i < files.length; ++i) {
+  const oldContent = fs.readFileSync(files[i]).toString();
+  const newContent = mdtoc.insert(oldContent, { bullets: '-' });
+  if (oldContent != newContent) {
+    console.log(`Updating ${files[i]}`);
+    fs.writeFileSync(files[i], newContent);
+  }
+}