Add exit bind function

Currently binding `exit` to a key checks too late that it's exitted,
so it leaves the shell hanging around until the user does an execute
or similar.

As I understand it, the `exit` builtin is supposed to only exit the
current "thread" (once that actually becomes a thing), and the
bindings would probably run in a dedicated one, so the simplest
solution here is to just add an `exit` bind function.

Fixes #7604.

doc_src/cmds/bind.rst

@@ -126,6 +126,8 @@ The following special input functions are available:
 
 - ``execute``, run the current commandline
 
+- ``exit``, exit the shell
+
 - ``forward-bigword``, move one whitespace-delimited word to the right
 
 - ``forward-char``, move one character to the right