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/package.json

@@ -0,0 +1,7 @@
+{
+  "name": "test-bun-hook",
+  "version": "1.0.0",
+  "bin": {
+    "test-bun-hook": "./bin/test-hook.js"
+  }
+}