This PR improves the keyboard navigation for the variable list. 
Before this PR, if you want to open/close nested variables, you had to
use the right/left & up/down arrow keys.
Now you can step through with just only using your left/right arrow
keys, this feels a bit more natural and more similar to how other
editors allow you to navigate through variables.

This PR also fixes the following issues:
- Allow selecting a scope to be the start of your selection
- Allow selecting previous item if the first item is selected

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https://github.com/user-attachments/assets/aff0b133-97be-4c09-8ee6-b11495ad5568

Release Notes:

- N/A