Add type overload for Push.Permission.request()

[Jicmou]

Fixes #243

[theLufenk]

@Jicmou Thank you for this.
There seems to be a test failing. Can you look into it please.

@Nickersoft LGTM. Can be merged.

[Jicmou]

@theLufenk Thanks for approving.

I can reproduce the failing CI on master. Run the following commands:

npm run test
// passing
npm run build
// builds sucessfully
npm run test
// failing

I ran the first two commands before commiting, as stated in the CONTRIBUTING.md, Travis ran the third.

I could submit my PR without the build though, as index.d.tshas no impact on the build.

[theLufenk]

@Nickersoft Over to you.

[theLufenk]

nudge @Nickersoft