Add Bun language support

Implements Bun as a new language option for pre-commit hooks, enabling
hooks to run using the Bun JavaScript runtime and package manager.

- Add bun.py language implementation with binary download/install
- Support system-installed Bun or automatic version download
- Add comprehensive tests including version handling and hook execution
- Register bun in all_languages.py
- Include test repository fixture for integration tests

testing/resources/bun-hook-repo/bin/test-hook.js

@@ -0,0 +1,16 @@
+#!/usr/bin/env node
+// Simple test hook that validates file content
+const fs = require('fs');
+
+const files = process.argv.slice(2);
+let failed = false;
+
+files.forEach(file => {
+  const content = fs.readFileSync(file, 'utf8');
+  if (content.includes('bad')) {
+    console.error(`Error in ${file}: contains 'bad'`);
+    failed = true;
+  }
+});
+
+process.exit(failed ? 1 : 0);