Prepack/clean script fails on Windows

[Venryx]

The prepack script fails on Windows, due to multiple issues:

1. Use of the rm command.

Solution: add shx as a dev-dependency, and have the script call shx rm instead: https://github.com/shelljs/shx

2. Use of ESM=1 to set an environment variable.

Solution: use cross-env: https://github.com/kentcdodds/cross-env

[Venryx]

Hmm, an issue is that dev-dependencies are not installed when running the prepack lifecycle script -- at least when installing using the npm install GithubUser/mst-decorators-type command.

Raised issue here asking for advice: shelljs/shx#168