Add updated type definitions

[t3hmrman]

I've done the bare minimum:

• Making use of the work in DefinitelyTyped's @types repository
• Replacing PromiseLike with Promise

I would love some help checking the published artifact however -- locally I'm using r2g and the build package looks fine (includes index.d.ts in the package root).

Resolves #259

[t3hmrman]

I could also add a basic r2g-based test to this PR and hook that into the project testing, but figured I'd at least get the regular PR up first.

[EDIT] - Quick & dirty local testing procedures:

First, Create a new small test project (npm init), with this index.ts:

import { createPool } from "generic-pool";
const pool = createPool({
  create: () => Promise.resolve(1),
  destroy: () => Promise.resolve(1),
});

pool
    .drain()
    // Expect Typescript to complain on the line below since PromiseLike has no .catch()
    .catch((err: Error) => {}); 

After creating the new project I did two things:

Confirming old (broken) functionality

• npm install generic-pool
• npm install @types/generic-pool (you may have to re-link the local generic-pool)
• tsc index.ts --watch
• confirm that typescript gives an error about .catch not being on PromiseLike

Checking the new index.d.ts

• Clone the branch
• Use r2g's Z stage to create a local published artifact @ $HOME/.r2g/temp/project (generic-pool will be in node_modules under that folder) -- roughly npm install -D r2g && ./node_modules/.bin/r2g --skip=t,s, it will fail but ignore it because the package does get packed)
• run npm link ~/.r2g/temp/project/node_modules/generic-pool inside the test project
• npm remove @types/generic-pool (you may have to re-link the local generic-pool)
• re-compile (tsc index.ts) and you should not see the same error

Again, this can be made more easily automatable with r2g (this is what it's for, you just need to add a folder called .r2g in the project and add tests in it. But the method above was doable for quick & dirty testing.