highlight: refactor to use stateful representation

This allows us to keep track of the source higlight groups,
and not only the final combined highlights.

src/nvim/api/vim.c

@@ -1851,3 +1851,22 @@ Object nvim_get_proc(Integer pid, Error *err)
 #endif
   return rvobj;
 }
+
+/// NB: if your UI doesn't use hlstate, this will not return hlstate first time
+Array nvim__inspect_cell(Integer row, Integer col, Error *err)
+{
+  Array ret = ARRAY_DICT_INIT;
+  if (row < 0 || row >= screen_Rows
+      || col < 0 || col >= screen_Columns) {
+    return ret;
+  }
+  size_t off = LineOffset[(size_t)row] + (size_t)col;
+  ADD(ret, STRING_OBJ(cstr_to_string((char *)ScreenLines[off])));
+  int attr = ScreenAttrs[off];
+  ADD(ret, DICTIONARY_OBJ(hl_get_attr_by_id(attr, true, err)));
+  // will not work first time
+  if (!highlight_use_hlstate()) {
+    ADD(ret, ARRAY_OBJ(hl_inspect(attr)));
+  }
+  return ret;
+}