JetBrains: Increase type-safety of JS -> Java bridge

Created by: philipp-spiess

I saw some unsafe typings and wanted to fix it. Going forward we should probably also encode the expected return type to avoid casting it.

This PR:

• Changes the Action type to be an enum instead of a loosely typed object.
• Changes the mock to use an exhaustive switch so that we get a type error when we add a new action that is not implemented (as opposed to a runtime error)

Test plan

• TS build works
• Here's an example of the exhaustive switch statement failing:

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.