decoration: split out "decoration" from "extmark" module

Decorations will only grow more complex. move the to a separate
file, so that extmark.c remains about extmarks.

src/nvim/api/private/helpers.h

@@ -7,6 +7,7 @@
 #include "nvim/vim.h"
 #include "nvim/getchar.h"
 #include "nvim/memory.h"
+#include "nvim/decoration.h"
 #include "nvim/ex_eval.h"
 #include "nvim/lib/kvec.h"