add a function for checking whether a tag is primitive or not

mainly for niche uses

src/dat/nbt.c

@@ -98,3 +98,17 @@ int nbt_prim_tagsize(u8 tag) {
 	default:      return -1;
 	}
 }
+
+int nbt_isprim(u8 tag) {
+	switch (tag) {
+	case NBT_I8:
+	case NBT_I16:
+	case NBT_I32:
+	case NBT_F32:
+	case NBT_I64:
+	case NBT_F64:
+		return 1;
+	default:
+		return 0;
+	}
+}