Add prepack script to package scripts

[DocX]

prepack is needed to be compatible with yarn when installing from github or local source. See yarn documentation: https://yarnpkg.com/advanced/lifecycle-scripts

prepack is also defined in npm: https://docs.npmjs.com/cli/v6/using-npm/scripts#life-cycle-scripts

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[brendandburns]

Looks fine to me, but we need you to sign the CLA.

Thanks!

[DocX]

I signed it

[brendandburns]

/lgtm
/approve

(apologies for missing the update on this one)

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: brendandburns, DocX

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