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 occured:
 - Error: 2u2 has an invalid reference: 'guidelines:regulation:9a25'
 - Error: got reference 'reguLations:regulation:B4' in 5b4, but 'reguLations' must be all lowercase
 - Error: got reference 'regulations:artiCle:14' in 10e1, but 'artiCle' must be all lowercase
 - Error: A5b++ has an invalid reference: 'guidelines:regulation:5a'
 - Error: got reference 'regulations:reGulation:A1a4' in H1b1+, but 'reGulation' must be all lowercase
 - Error: Rule 9a25 is referenced in ['2u2'], but was not found
 - Error: Article 14 is referenced in ['10e1'], but was not found