Add TypeScript union string type for custom event names

[cngJo]

Description

This adds more type-safety to extension developers, since the event name in onEvent is now no longer of type string, but uses a. new union-type of all available event names as string.

The type and the available events are based on what's written in the docs.

Corresponding issue: there is no issue atm. please let me know when I should create one.

Testing

This only changes the type definition, not the actual source-code, therefore no tests are provided.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded

[alexpetros]

Looks good, thanks!