Closes #8791 
Discussion: #8657 

### Summary
This adds the FocusTerminalIntent App Intent and related function
(focusSurface), allows external tools (such as Shortcuts/Siri) to
programmatically move focus to a specific terminal window or tab.

### Verification
This functionality has been tested across following scenarios,
confirming correct focus behavior for:

- Split Window
- Tab Group
- Quick Terminal

### Note
It is not supported to move focus to a split that is hidden by a zoomed
split. The same applies to the CloseTerminalIntent.

### AI Disclosure
This pull request was made with assistance from Claude Code.
I reviewed all AI-generated code and wrote the final output manually.

Config contains the command, working directory, and environment
variables intended to be passed to the new split, but it looks like we
forgot to include it as an argument in this branch.

Discussion: https://github.com/ghostty-org/ghostty/discussions/8637

This has no meaningful functionality yet, it was one of the paths I was
looking at for #8505 but didn't pursue further. But I still think that
this makes more sense in general for the macOS app and will likely be
more useful later.

SwiftUI's ImageRenderer must not be called outside the main thread.

The `@MainActor` annotation is only relevant for our own code, not
for calls from frameworks. The machinations around Shortcuts end up
calling the displayRepresentation method outside the main thread.

By capturing the screenshot as NSImage, all data is retained and can
be processed outside the main thread.

This fixes an Apple Shortcuts crash for macOS 15 and earlier.
Unfortunately it looks like we can't guard these with `@available`. I'm
going to report an Apple Feedback about this but for now this gets
shortcuts working on macOS 15 and earlier.

Fixes #7647

See #7647 for context. This commit works by extending the `input` work
introduced in #7652 to libghostty so that the macOS can take advantage
of it. At that point, its just the macOS utilizing `input` in order to
set the command and `exit` up similar to Terminal and iTerm2.