patch 8.1.1276: cannot combine text properties with syntax highlighting

Problem:    Cannot combine text properties with syntax highlighting.
Solution:   Add the "combine" field to prop_type_add(). (closes #4343)

src/structs.h

@@ -727,6 +727,7 @@ typedef struct proptype_S
 
 #define PT_FLAG_INS_START_INCL	1	// insert at start included in property
 #define PT_FLAG_INS_END_INCL	2	// insert at end included in property
+#define PT_FLAG_COMBINE		4	// combine with syntax highlight
 
 // Sign group
 typedef struct signgroup_S