Rename QueryResult to MysqlResult

[SimonFrings]

In this pull request, I propose changing the current QueryResult to MysqlResult. This change aligns with the recently introduced MysqlClient from #186, ensuring a consistent "Mysql" prefix throughout.

The name "QueryResult" is not incorrect in this case. However, when working with multiple database clients in one project, it can lead to confusion about which specific "query result" is being referred to. For instance, the https://github.com/clue/reactphp-sqlite project has a similar class named Result, which should also be renamed to something like SqliteResult in the future.

Furthermore, we applied a similar reasoning in reactphp/socket#263 and reactphp/http#417.

Refs #147
Builds on top of #186

[WyriHaximus]

While I think React\MySQL\Result would be better than React\MySQL\MysqlResult it's not a deal breaker for me. And I do agree with the argument that it makes low-level usage of this package easier. And honestly it's going tobe abstracted away by an ORM anywa.

[clue]

@SimonFrings Thanks for looking into this.

I agree that either name is debatable, but including the Mysql prefix definitely helps to avoid some name clashes, so let's go ahead with this!