Add default option to close "Split"

[el22or]

Problem:

At the moment, there is a problem to close Split once you create it (on Linux).

There is no GUI option to close Split...

I would expect it next to "Split Right" and "Split Down" actions.

I was also looking for default keybinding option and could not find one.

So, to close split from a surface I had to close the tab. And so the other splits with it too.

Proposed solution:

There should be GUI option "Close Split", and to close/remove focused split from a surface.
There should be default keybinding for the same action.

[el22or]

^D works well for closing focused Split.

[rhodes-b]

You should suggest that here #4952
it is already adding quite a bit of menu stuff

[rhodes-b]

There should be default keybinding for the same action.

There is a default binding to close surface which applies to splits and tabs
https://github.com/ghostty-org/ghostty/blob/main/src/config/Config.zig#L2449-L2452
but right now it overrides close surface which unbinds it unintentional from
#4033

note this only effects the main branch if you are on stable release 1.0.1 (newest) the close tab keybind was not added yet