Adjust zed -n behavior

[eholk]

When there's already a workspace that matches the paths provided, activate that workspace, otherwise create a new window. This is different from the default behavior of always adding to the sidebar.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Release Notes:

• Changed zed -n to only open a new window for folders that don't already exist as a workspace root in some window. If the appropriate window already exists, -n will activate it instead.