ex_getln: Make use of new parser to color expressions

Retires g:Nvim_color_expr callback.

src/nvim/viml/parser/parser.c

@@ -0,0 +1,13 @@
+#include "nvim/viml/parser/parser.h"
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "viml/parser/parser.c.generated.h"
+#endif
+
+
+void parser_simple_get_line(void *cookie, ParserLine *ret_pline)
+{
+  ParserLine **plines_p = (ParserLine **)cookie;
+  *ret_pline = **plines_p;
+  (*plines_p)++;
+}