Add hover for global variables

[snutij]

Motivation

Iteration of #2644
Handle global variables in hover requests.

Implementation

Handle each kind of global variable node in the listener pattern, then search for entries in the index with the variable name, and finally, push the documentation for entries to the response_builder.

Automated Tests

• Added a test with assertions for each kind of global variable
• Added a test with assertions to ensure the target is unset if the hover is not exactly on the variable name

Manual Tests

• Write $DEBUG within a file, then hover over it. You should see documentation from the RBS declaration.
• Write a custom $foo variable with a comment, hover over it, you should see the hover.
• You should not see documentation when hovering not exactly on variable name

Kapture.2024-10-16.at.09.42.12.mp4

[vinistock]

This is great! Thanks for the contribution 🚀