feat(webchat): allow users to subscribe to different webchat events

[EFF]

Done:

• converted inject.js to typescript ==> note that I didn't change the code much, I didn't want to go for a full re-implementation
• implemented event filtering and handling logic in the already existing window message listener in inject.ts ==> this not only improves the user experience it also prevent the user to create multiple window event listeners
• converted event types to types that are properly categorized
• fixed some typings along the way

[laurentlp]

Note: The webchat test on firefox never completes on the CI but was tested locally and ran with success!