decor: sketch new decorations API

return decorations back

lol no nvim_buf_get_virtual_text

share decorations that are hl only to avoid alloc avalanche

src/nvim/main.c

@@ -21,6 +21,7 @@
 #include "nvim/ex_cmds.h"
 #include "nvim/ex_cmds2.h"
 #include "nvim/ex_docmd.h"
+#include "nvim/extmark.h"
 #include "nvim/fileio.h"
 #include "nvim/fold.h"
 #include "nvim/getchar.h"
@@ -160,6 +161,7 @@ void early_init(mparm_T *paramp)
   env_init();
   fs_init();
   handle_init();
+  extmark_init();
   eval_init();          // init global variables
   init_path(argv0 ? argv0 : "nvim");
   init_normal_cmds();   // Init the table of Normal mode commands.