feat: check references to rules and articles

[Nanush7]

Added the ReferenceCheck visitor:

• Flag rules referencing other non-existent/removed nodes.
• Flag invalid references (e.g. regulations:guideline:..., guidelines:regulation:... or REgulations:...:...).

An example output with errors:

Couldn't compile file, the following occurred:
 - Error: 9t2 has an invalid reference: 'rgulations:regulation:9m2'
 - Error: Rule 4f is referenced in ['11e1', '11e1+'], but was not found
 - Error: Rule E2b++ is referenced in ['1h+++'], but was not found
 - Error: Rule 4f+ is referenced in ['11e1+'], but was not found

Note:
Given that we want to check cross-references between regulations and guidelines (who live in different ASTs), we need to keep the state of the ReferenceCheck class after each WCADocument has been visited. The obvious way to do this is to use a Singleton. I think it makes sense to do it this way, in order to minimize the changes required to the WCAParser class. Moreover, there's additional logic that needs to run when the parser gets an instance of the ReferenceCheck visitor class (see the ReferenceCheck.get_instance method).