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/eval.h

@@ -165,6 +165,8 @@ typedef enum {
   VV__NULL_DICT,  // Dictionary with NULL value. For test purposes only.
   VV__NULL_BLOB,  // Blob with NULL value. For test purposes only.
   VV_LUA,
+  VV_RELNUM,
+  VV_WRAP,
 } VimVarIndex;
 
 /// All recognized msgpack types