Make document generic

[vinistock]

Motivation

This is the final step for separating document concerns. In order to support other document types like RBS, we need to ensure that the base Document class is generic and accounts for different types of @parse_result.

Implementation

This PR:

• Makes Document generic
• Fixes the type of the parse result in ERBDocument and RubyDocument
• Starts returning early from requests depending on document types, since we now need to properly narrow types to satisfy type checking
• Moves locate outside of the parent Document since the implementation is Ruby specific (but also used in ERBDocument)

It's a rather large PR, but the changes are all interdependent and I couldn't split it into smaller chunks.

Automated Tests

Existing tests cover the refactor.

[andyw8]

🎆