Add completion for global variables

[snutij]

Motivation

Iteration of #2644

Handle global variables in completion requests.

Implementation

• Add $ as a new trigger_character (see this comment)
• Add listener for all global variable nodes
• Find in index with prefix
• Push entries as CompletionItem in response_builder

Automated Tests

• Added a test with assertions for each kind of global variable, and RBS completions.

Manual Tests

• Write $ within a file, you should see completions coming from the RBS declaration.
• Write $foo = 1, then bellow $f, you should see the completion.

Kapture.2024-10-21.at.23.31.16.mp4

[vinistock]

Thank you for the contribution!