[p-queue] introduce typings

[BendingBender]

Based on typings proposed in sindresorhus/p-queue#6

• Use a meaningful title for the pull request. Include the name of the package modified.
• Test the change in your own code. (Compile and run.)
• Follow the advice from the readme.
• Avoid common mistakes.
• Run npm run lint package-name (or tsc if no tslint.json is present).

If adding a new definition:

• The package does not provide its own types, and you can not add them.
• If this is for an NPM package, match the name. If not, do not conflict with the name of an NPM package.
• Create it with dts-gen --dt, not by basing it on an existing project.
• tslint.json should be present, and tsconfig.json should have noImplicitAny, noImplicitThis, and strictNullChecks set to true.

[dt-bot]

types/p-queue/index.d.ts

Checklist

• is correct naming convention?
  • https://www.npmjs.com/package/p-queue - https://github.com/sindresorhus/p-queue#readme
• has a test file? (p-queue-tests.ts or p-queue-tests.tsx)
• pass the Travis CI test?