Lightweight module system setup for shared code

[dfcowell]

This PR takes a very light touch to introducing yarn workspaces v1 and a shared module in the "common" directory.

Basically the same as my last PR (#17) except it doesn't completely rearrange the project directories.

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[dfcowell]

Just a couple of comments on this one.

1. Workspaces hoists the yarn.lock file into the root directory and it contains all dependencies for all packages in the workspace, that's why the existing lockfiles were removed.
2. I shuffled some small bits of shared config around into the root directory.

[b3nk3]

This is a very neat approach. Clear and concise.