refactor: unifies spread types in their proper places

[wellwelwel]

This PR is the last part of the TypeScript refactoring (#2072).
As before in #2067, the final usage is unaffected by these changes.

By separating the PRs, I believe it's now much better to review the changes 🔬

---

Unifying the types

• I'll use the Pool as example
• The same idea goes for all the other types that have been spread out

All Pool types spread across the index.d.ts, typings/mysql/index.d.ts and typings/mysql/lib/Pool.d.ts have been unified in "typings/mysql/lib/Pool.d.ts".

---

About the approach of reusable query and execute overloads

• I'll use the execute as example
• The same idea goes for the query overloads

I moved all variations (overloads) of the execute around the types into "typings/mysql/lib/protocol/sequences/ExecutableBase.ts".

Pool as example is a class, not an interface, so I did the ExecutableBase as an Anonymous Class.

I chose this method because it allows to dynamic extending the QueryableBase and Emitter like a Function.

For example:

declare class Pool extends QueryableBase(ExecutableBase(EventEmitter)) { /* ... */ }

It doesn't changes the execute or query types, just where they come from.

How this works in practice:

Before

After

---

Notes

• No types were added or removed, only unified 🙋🏻‍♂️

[wellwelwel]

@sidorares, if you think it's better, I can do a test to strictly check the returned types for each overload of execute and query.

These tests wouldn't be in the same way as the existing tests for the execute and query.

---

Since it's just a refactoring, I didn't focus on tests, but I believe this could be a safer approach.

[wellwelwel]

After thinking for a while, I decided to create a strict test to check each overload and its returned types of execute and query.

This will ensure that both this refactoring and any future changes to the execute and query methods are tested for variations of usage and their corresponding returns in each variation of usage 🤹🏻‍♀️

These tests aren't the same as the existing tests for the execute and query.

Now I can say that this part of the #2072 is completed.
That will make the fix and the creation of the promise-based PoolCluster typings much easier 🎉

---

@sidorares, in this refactoring, if approved, I would like you to handle the merge.