path selector - update behavior of select_mode=single

This commit changes the behavior of the 'single' select mode,
where a user can only have 1 environment selected at a time.

Example:
- assume dev, test, prod exist
- dev is selected

Scenario: User wants to select 'test'

Before the change:
- unselect dev and then select test

After the change
- select test (and dev is automatically unselected)

After the change: