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

src/input.cpp

@@ -152,6 +152,7 @@ static const input_function_metadata_t input_function_metadata[] = {
     {readline_cmd_t::func_or, L"or"},
     {readline_cmd_t::expand_abbr, L"expand-abbr"},
     {readline_cmd_t::delete_or_exit, L"delete-or-exit"},
+    {readline_cmd_t::exit, L"exit"},
     {readline_cmd_t::cancel_commandline, L"cancel-commandline"},
     {readline_cmd_t::cancel, L"cancel"},
     {readline_cmd_t::undo, L"undo"},

src/input_common.h

@@ -75,6 +75,7 @@ enum class readline_cmd_t {
     func_or,
     expand_abbr,
     delete_or_exit,
+    exit,
     cancel_commandline,
     cancel,
     undo,

src/reader.cpp

@@ -3076,6 +3076,13 @@ void reader_data_t::handle_readline_command(readline_cmd_t c, readline_loop_stat
             delete_char();
             break;
         }
+        case rl::exit: {
+            // This is by definition a successful exit, override the status
+            parser().set_last_statuses(statuses_t::just(STATUS_CMD_OK));
+            exit_loop_requested = true;
+            check_exit_loop_maybe_warning(this);
+            break;
+        }
         case rl::delete_or_exit:
         case rl::delete_char: {
             // Remove the current character in the character buffer and on the screen using