Add commit guide to CONTRIBUTING.md

With a reference link for contributors to better understand conventional commits, and the commit emojis.

And a table for quick copy + paste for commit prefixes.

Here's all the emojis I could find in the [PRs](https://github.com/GodotModding/godot-mod-loader/pulls).

**Please edit this text if you know what these are for.**

| Type | Prefix | Notes |
| --- | --- | --- |
| Feature | `feat: ✨` | Added new feature(s) |
| Refactor | `refactor: ♻️` | Reworked existing code to be better (eg. for clarity/performance) |
| Refactor | `refactor: 🚚` | Move files / code (eg #255) |
| Bugfix | `fix: 🐛` | Fixed an error in the code |
| Bugfix | `fix: ✏️` | Fixed a typo |
| Style | `style: 🎨` | Improve the code style of the code |
| Style | `style: 🏷️ ` | Added / Updated types |
| Docs | `docs: 📝` | Related to writing docs (eg #279) |
| Tests | `test: ✅` | Add / Update tests |
| Build | `build: 🔥` | Changes that affect the build system or external dependencies - The 🔥stands for removing code or files, so in this case, a file or code related to the build system was removed. |
| CI | `ci:` | Related to continuous integration (CI) |
| Chore | `chore: ` | Other changes that don't modify src or test files |
| Chore | `chore: ⬆️` | Update a version number |