feat(tool-window): close other tool windows when opening one

[CalvinAllen]

Summary

• When a tool window button is clicked to open, all other tool windows configured in the launch bar are automatically hidden first
• Provides Rider-like exclusive tool window behavior where only one tool window is visible at a time
• Toggling (hiding) a tool window still works as before without affecting others

Closes #7

Test plan

• Click Solution Explorer button — it opens, other configured tool windows close
• Click Git Changes while Solution Explorer is open — Solution Explorer hides, Git Changes opens
• Click an already-open tool window — it hides (toggle still works)
• Non-tool-window items (VsCommand, ExternalProgram) are unaffected