Move Atom/Lens-only version to focal-atom

[oleksiilevzhynskyi]

It's a second attempt to move Atom/Lens-only version into a separate package.

Package @grammarly/focal-atom now contains only atoms/lenses-related code, without any dependency on React. @grammarly/focal keeps the same interface but depends on @focal-atom - no breaking changes here.

Just to keep it simple, I propose to release both packages always together and under the same version (https://github.com/facebook/react/releases releasing this way)

NOTE: "contributors" were moved into AUTHOR file (AUTHOR uses as the default value for "contributors" https://docs.npmjs.com/files/package.json#default-values)

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TBD:

• [ ] ~Separate Readme for @grammarly/focal-atom~
• [x] Polish release script

Resolves #68