Add `prepare` script that builds the library

https://docs.npmjs.com/cli/v7/using-npm/scripts#life-cycle-scripts

This should allow installing from a git commit, and will also provide better assurance that `yarn build` always gets run before `npm publish`.

Start with the npm lifecycle scripts documentation linked in the issue and inspect the project's existing `yarn build` and `npm publish` workflow. Add the requested `prepare` lifecycle behavior, then verify that installing from a Git commit builds the library and that publishing runs the build first.

Written by the indexing model from the issue text.