Support for setting transaction isolation level

[WiktorKryzia]

Hello, this driver for Kysely currently does not support changing the isolation level, which can be done using Kysely's TransactionBuilder.setIsolationLevel() method.
The name of isolation level placed in the object { isolationLevel: '...' } is passed by Kysely to the method PlanetScaleDriver.beginTransaction as a second parameter (usually named "settings").
In this case, it should be further passed to the PlanetScaleConnection.beginTransaction and used there, for example, in such a way:

if (settings.isolationLevel) {
    await this.#transactionConn.execute(`SET TRANSACTION ISOLATION LEVEL ${settings.isolationLevel}`);
}

[jacobwgillespie]

Hey, thanks for flagging this! Feel free to open a PR to fix if you'd like, otherwise I'll get around to updating things shortly.