syntax.c: register 'Normal' highlight group

- :hi Normal works with -u NONE
- Makes HL_TABLE and ATTR_ENTYRY a function instead of a macro so that in can be used in gdb.
- Introduces ATTRENTRY_INIT to init attrentry_t

src/nvim/syntax_defs.h

@@ -73,4 +73,14 @@ typedef struct attr_entry {
   int cterm_fg_color, cterm_bg_color;
 } attrentry_T;
 
+#define ATTRENTRY_INIT { \
+  .rgb_ae_attr = 0, \
+  .cterm_ae_attr = 0, \
+  .rgb_fg_color = -1, \
+  .rgb_bg_color = -1, \
+  .rgb_sp_color = -1, \
+  .cterm_fg_color = 0, \
+  .cterm_bg_color = 0, \
+}
+
 #endif // NVIM_SYNTAX_DEFS_H