Migrate to a shared pub workspace

[parlough]

• Adds a root package to act as the root of the workspace.
• Removes usages of package:mono_repo.
• Adds package:melos as a dev dependency of the workspace root.
• Writes a new GitHub actions workflow to verify formatting, analysis, and tests across the workspace (using melos).
• Updates the tests in pkgs/dash_design which have been failing but weren't running.

This required some updates to the dash_design package tests.