This adds two new propeties to make it easy to get the frontmost (main)
window and the focused terminal within a tab. We already had a property
to get the selected tab of a tab group.
Add a `surface configuration` record type to the scripting dictionary,
implement `new surface configuration` (with optional copy-from), and allow
`new window` to accept `with configuration`.
Add a `new window` command to the scripting dictionary and wire it to
`NSApplication` so AppleScript can create Ghostty windows.
The command returns a scripting `window` object for the created window,
with a fallback to a direct wrapper when AppKit window ordering has not
yet refreshed in the current run loop.
