Support npm7

[falsandtru]

Fixes #273

@cartant Can you merge?

[cartant]

I'm not going to merge this.

You have an issue because you are using a specific, pre-release version of TypeScript and I guess that npm - unlike Yarn - doesn't provide a mechanism - like resolutions - to deal with the fact that it deems the version to not match the (wide!) semver range.

AFAICT, the point of the changes npm version 7 was not to force packages to removal their peerDependencies. Maybe that was their intention, but I doubt it. Anyway, that would require further investigation and I don't have the time for that, ATM.

Maybe you should use npm's --legacy-peer-deps option? ATM, I don't think removing peerDependencies is the solution.

[falsandtru]

I feel the current peerDependencies feature is different from the feature we need. So I think you don't have to keep peerDependencies. So if you can't find the correct solution, I think peerDependencies should be removed anymore.