Streamline npm publish and Now deploy files

[shawnbot]

Fixes #241. This does a couple of things:

1. Removes .npmignore
2. Adds a files field to package.json that whitelists the dist and src directories and then blacklists src/__tests__ and src/utils, since these are only used by tests
3. Removes a couple of files from now.json so that they're not deployed
4. Moves our jest configuration out of package.json and into jest.config.js, reducing the size of the file that gets published.
5. Adds a shell script script/npm-pack-check, which checks that the certain files are and are not included in the tarball that gets published to npm. (There's probably an npm module for this, but I couldn't find one after a cursory search, and would be happy to extract this later for use in other Primer repos.)

TODO

• Review
• Merge Make static assets available on path alias #238
• Update base branch to release-2.0.2-beta

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[shawnbot]

@emplums this was intended to merge into #238, but because it touches npm publish logic it might be better to merge into our next release branch.

[emplums]

Looks good to me!