Add a new `split` command to the AppleScript scripting dictionary that
splits a terminal in a given direction (right, left, down, up) and
returns the newly created terminal.
The command is exposed as:
split terminal <terminal> direction <direction>
Also adds a `fourCharCode` String extension for converting four-character
ASCII strings to their FourCharCode (UInt32) representation.
We already had an established Ghostty.Shell namespace (previously a
struct; now a more idiomatic enum), and locating these functions next to
each other makes it clearer how they relate to one another.
When we're building an input string that's explicitly meant to be used
as a shell command, quote (escape) it using the same logic as Python's
shlex.quote function.
This specifically addresses issues we've seen when open(1)'ing Ghostty
with filename arguments that contain spaces.
