feat: before: and after: event, plus exported event names

[daogrady]

Stacked on #110
Fixes #103

• introduce a second event that gives users the chance to modify the source CSN before the OpenAPI compilation takes place
• Exposes events as top level API, containing the two event names for easier and more consistent access to these names

[daogrady]

@chgeo
We'd like to provide two hooks for users: before the compilation to OpenAPI takes place, so users can modify the input CSN, as well as one after, so users can modify the result before it is written to file.
Do we have naming conventions for this two-phase hook approach?
This PR introduces these events as before:compile.to.openapi and after:compile.to.openapi, but we can still change the nomenclature here.

[tim-sh]

Looks good, apart from the 'after' event name, as discussed elsewhere

[tim-sh]

⏰ Afterwards, redo #111