CI test with DefinitelyTyped

[saschanaz]

Sometimes compatibility with DefinitelyTyped breaks, a test will be great to prevent this.

[saschanaz]

Currently I think the CI script may do:

1. Download DefinitelyTyped in ./DefinitlyTyped/ and do npm install there which will install a new TypeScript copy
2. Replace ./DefinitelyTyped/node_modules/typescript/lib.dom.d.ts and lib.dom.iterable.d.ts with the new generated ones
3. Do npm test in ./DefinitelyTyped.

[mhegazy]

Running all tests on DT takes a while. we do have a nightly test running on DT that catches issues once they make it to the nightly build.
You can also manually run the tests using the DT test runner in the main ts repo.

[saschanaz]

A manual test is always possible, but that still allows accidental merging of erroneous PRs. Also, it's not very easy for every contributor.