build(): make npm install also install typings

[jhades]

This simplifies the installation process, we get one less step. This would help avoid erroneous bug reports due to missing typings

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[jhades]

I signed it!

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!

[jelbourn]

@hansl has opinions about postinstall scripts.

[hansl]

Now that we use separate packages for every components, this is okay.

LGTM.

[hansl]

Committed with ead66f7

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