syntax: Use RGB/GUI attribute information for "abstract_ui"

Instead of using classic cterm color numbers and attributes, treat "abstract_ui"
as a GUI: Pass rgb color numbers and gui attributes when the "highlight_set" UI
method is called. The terminal UI will have to translate RGB color information
to an appropriate color number, and the "term"/"cterm" :highlight keys will
eventually be deprecated.

src/nvim/api/vim.c

@@ -21,6 +21,7 @@
 #include "nvim/message.h"
 #include "nvim/eval.h"
 #include "nvim/misc2.h"
+#include "nvim/syntax.h"
 #include "nvim/term.h"
 #include "nvim/getchar.h"
 #include "nvim/os/input.h"
@@ -546,6 +547,11 @@ void vim_unsubscribe(uint64_t channel_id, String event)
   channel_unsubscribe(channel_id, e);
 }
 
+Integer vim_name_to_color(String name)
+{
+  return name_to_color((uint8_t *)name.data);
+}
+
 Array vim_get_api_info(uint64_t channel_id)
 {
   Array rv = ARRAY_DICT_INIT;