Summary: Now that the Dagit CRA is ejected, remove `react-scripts` from `@dagit/core`. It currently uses it for running Jest, but we can just set that up ourselves.

Test Plan: Buildkite, run `yarn why react-scripts` to verify that it's gone.

Reviewers: bengotow

Reviewed By: bengotow

Differential Revision: https://dagster.phacility.com/D7672