refactor: use vfs=memdb instead of mode=memory&cache=shared

Shared cache is discouraged. The supported alternative
(and increasingly recommended by SQLite developers),
is the memdb VFS (which I believe is supported by all drivers).
I'm sure modernc has it, and mattn also does if compiled with
SQLITE_ENABLE_DESERIALIZE, which became the default in 2021.

https://github.com/ncruces/go-sqlite3/issues/94#issuecomment-2157679766

internal/command/string/string_test.go

@@ -9,7 +9,7 @@ import (
 
 func getDB(tb testing.TB) (*redka.DB, redis.Redka) {
 	tb.Helper()
-	db, err := redka.Open(":memory:", nil)
+	db, err := redka.Open("file:/data.db?vfs=memdb", nil)
 	if err != nil {
 		tb.Fatal(err)
 	}