feat(ui): add 'statuscolumn' option

Problem:    Unable to customize the column next to a window ('gutter').
Solution:   Add 'statuscolumn' option that follows the 'statusline' syntax,
	    allowing to customize the status column. Also supporting the %@
	    click execute function label. Adds new items @C and @s which
	    will print the fold and sign columns. Line numbers and signs
	    can be clicked, highlighted, aligned, transformed, margined etc.

src/nvim/option.h

@@ -19,6 +19,8 @@ typedef enum {
 #define BCO_ALWAYS      2       // always copy the options
 #define BCO_NOHELP      4       // don't touch the help related options
 
+#define MAX_NUMBERWIDTH 20      // used for 'numberwidth' and 'statuscolumn'
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "option.h.generated.h"
 #endif