fix: PoolConnection redundancy when extending Connection interface in TypeScript

[wellwelwel]

As I mentioned in #1722 issue:

Once the PoolConnection.d.ts has already declared extending the Connection interface:

declare class PoolConnection extends Connection {
    connection: Connection;
    release(): void;
}

Then it doesn't needs to re extend the Connection in index.d.ts #86 like:

export interface PoolConnection extends mysql.PoolConnection, Connection {
  promise(promiseImpl?: PromiseConstructor): PromisePoolConnection;
}

Causing the error:

[...] Interface 'PoolConnection' cannot simultaneously extend types 'PoolConnection' and 'Connection'.
  Named property 'execute' of types 'PoolConnection' and 'Connection' are not identical.

86 export interface PoolConnection extends mysql.PoolConnection, Connection {
                    ~~~~~~~~~~~~~~

---

So, I just removed the Connection from extends in index.d.ts #86 and it will work:

export interface PoolConnection extends mysql.PoolConnection {
  promise(promiseImpl?: PromiseConstructor): PromisePoolConnection;
}

---

As you recommended in #1722:
For this PR, I cloned the master and tested it again in a real use (locally), it works perfectly.

---

Extra tests:

• "skipLibCheck": false,

• "skipLibCheck": true,

• Also, without skipLibCheck.

[wellwelwel]

@sidorares Please, let me know if you have any questions or want to change anything 👨🏻‍💻

[sidorares]

@wellwelwel thanks for the PR! I'm away with no internet connection for the next 3 days but I'll try to test and merge as soon as I'm back

[sidorares]

Actually I think this can be merged right now, LGTM