bindings: add the "flippipe" bindable function

So the toggle for piping to an external command can be rebound.

This fixes https://savannah.gnu.org/bugs/?53987.

doc/nano.texi

@@ -1312,6 +1312,10 @@ Toggles between searching for text and targeting a line number.
 @item flipexecute
 Toggles between inserting a file and executing a command.
 
+@item flippipe
+When executing a command, toggles whether the current buffer (or marked
+region) is piped to the command.
+
 @item flipnewbuffer
 Toggles between inserting into the current buffer and into a new
 empty buffer.

doc/nanorc.5

@@ -665,6 +665,10 @@ Toggles between searching for text and targeting a line number.
 .B flipexecute
 Toggles between inserting a file and executing a command.
 .TP
+.B flippipe
+When executing a command, toggles whether the current buffer (or marked
+region) is piped to the command.
+.TP
 .B flipnewbuffer
 Toggles between inserting into the current buffer and into a new
 empty buffer.

src/global.c

@@ -1651,6 +1651,8 @@ sc *strtosc(const char *input)
 		s->func = backup_file_void;
 	else if (!strcasecmp(input, "flipexecute"))
 		s->func = flip_execute;
+	else if (!strcasecmp(input, "flippipe"))
+		s->func = flip_pipe;
 #endif
 #ifdef ENABLE_MULTIBUFFER
 	else if (!strcasecmp(input, "flipnewbuffer"))