bindings: add the 'linenumbers' bindable function

This makes the line-numbers toggle rebindable, like all the others.

doc/nano.texi

@@ -1359,6 +1359,9 @@ Toggles smooth scrolling (when moving around with the arrow keys).
 @item softwrap
 Toggles the displaying of overlong lines on multiple screen lines.
 
+@item linenumbers
+Toggles the display of line numbers in front of the text.
+
 @item whitespacedisplay
 Toggles the showing of whitespace.
 

doc/nanorc.5

@@ -715,6 +715,9 @@ Toggles smooth scrolling (when moving around with the arrow keys).
 .B softwrap
 Toggles the displaying of overlong lines on multiple screen lines.
 .TP
+.B linenumbers
+Toggles the display of line numbers in front of the text.
+.TP
 .B whitespacedisplay
 Toggles the showing of whitespace.
 .TP

src/global.c

@@ -1667,6 +1667,10 @@ sc *strtosc(const char *input)
 			s->toggle = SMOOTH_SCROLL;
 		else if (!strcasecmp(input, "softwrap"))
 			s->toggle = SOFTWRAP;
+#ifdef ENABLE_LINENUMBERS
+		else if (!strcasecmp(input, "linenumbers"))
+			s->toggle = LINE_NUMBERS;
+#endif
 		else if (!strcasecmp(input, "whitespacedisplay"))
 			s->toggle = WHITESPACE_DISPLAY;
 #ifdef ENABLE_COLOR