[dagit] Warn on tab removal in Playground
Stacked on https://dagster.phacility.com/D4747.
Add a warning dialog when a tab is about to be removed on the Playground. This prevents accidental deletion of tabs and loss of configuration work.
Test Plan: View Playground, remove tab. Verify proper dialog rendering and behavior.
Reviewers: bengotow, alangenfeld, prha, sashank
Reviewed By: bengotow
Differential Revision: https://dagster.phacility.com/D4748