ast: Support more annotation scopes

This commit updates the parser and checker to support additional
annotation scopes:

- `document` for attaching metadata at the document-level which covers
  all rules with the same name in the package.

- `package` for attaching metadata at the package-level which covers
  all rules in the same package.

- `subpackages` which is the same `package` but also applies to all
  subpackages (recursively).

This commit also makes the 'scope' field optional in the METADATA
block. If not specified, the scope will default to the attached node.

Signed-off-by: Torin Sandall <torinsandall@gmail.com>