Expose terminal surfaces as elements on both ScriptWindow and ScriptTab,
allowing AppleScript to enumerate terminals scoped to a specific window
or tab (e.g. `terminals of window 1`, `terminals of tab 1 of window 1`).
Changes:
- Add `<element type="terminal">` to window and tab classes in Ghostty.sdef
- Add `terminals` computed property and `valueInTerminalsWithUniqueID:`
lookup to ScriptWindow (returns all surfaces across all tabs)
- Add `terminals` computed property and `valueInTerminalsWithUniqueID:`
lookup to ScriptTab (returns surfaces within that tab)