ex_getln: Make use of new parser to color expressions

Retires g:Nvim_color_expr callback.

src/nvim/mbyte.h

@@ -60,6 +60,12 @@ typedef enum {
   CONV_ICONV     = 5,
 } ConvFlags;
 
+#define MBYTE_NONE_CONV { \
+  .vc_type = CONV_NONE, \
+  .vc_factor = 1, \
+  .vc_fail = false, \
+}
+
 /// Structure used for string conversions
 typedef struct {
   int vc_type;  ///< Zero or more ConvFlags.