Share lib script with npm postinstall script

[code-asher]

1. Change lib.sh to use sh to match the postinstall script.
2. Will need to change some syntax like remove local. Will need to do something about pushd/popd as well. (Alternatively we could move the arch and asar methods into a separate shared file).
3. Include lib.sh in the npm package.
4. Source lib.sh in the postinstall and remove the duplicated methods.
5. arch method is slightly different so may need to make some changes there.

[stale]

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