Allow :x on empty Helix

[m13253]

Related issue:
About mapping ZZ to :x: #2041 (comment)

Steps to reproduce:

1. Use the keybind config:

   [keys.normal]
   Z = { Q = ":quit!", Z = ":x" }

2. Open an empty Helix:

   $ hx

3. Type ZZ to quit.

What to expect:
Helix should exit.

What to observe:
Error message: cannot write a buffer without a filename

Workaround
In Vim, a similar keybinding is called ZQ, which triggers :quit!.
You can use ZQ in this situation.

Why ZZ doesn't work in Helix?
There is a difference in Helix's :x command comparing with Vim's.
In Vim:

• :wq means save and quit, no matter the file has been modified or not.
• :x (:xit) means if the file has been modified, save and quit, otherwise directly quit.
• For an empty unnamed buffer, :x works without a filename, while :wq doesn't.

However in Helix:

• :x is an alias to :wq.
• Both save and quit, even with no changes or with external changes.
• For an empty unnamed buffer, both don't work without a filename.

Proposed solutions
Make :x different from :wq.
Besides the benefit of being able to :x an empty unnamed buffer, the original behavior of :x is important when the file is changed from another application: You usually don't want to overwrite that change.

[raygervais]

I'd like to take a crack at this, it's a small quality of life improvement that I believe will also help my understanding of learning Rust in the perspective of a grand project