Add the test_regexp.c module containing a cross-platform implementation

of the REGEXP operator.

FossilOrigin-Name: 46c8c01b751c1ea7fc02cc35e3b5bb99dbe46c4b

src/shell.c

@@ -1479,6 +1479,12 @@ static void open_db(struct callback_data *p){
     }
 #ifndef SQLITE_OMIT_LOAD_EXTENSION
     sqlite3_enable_load_extension(p->db, 1);
+#endif
+#ifdef SQLITE_ENABLE_REGEXP
+    {
+      extern sqlite3_add_regexp_func(sqlite3*);
+      sqlite3_add_regexp_func(db);
+    }
 #endif
   }
 }