chore(changelog): Mention `gleam cleam` command.
Nothing rhymes better with `clean` like `gleam`.
## Unreleased
+- You can now remove build artifacts using the new `gleam clean` command.
- The `compile-package` can now generate `package.app` files and compile source
modules to `.beam` bytecode files.
- The flags that `compile-package` accepts have changed.
