Migrate typeEnum function from a built-in into an oas custom function

[nulltoken]

Chore summary
While working on #913, I tried to implement typedEnum as a custotom function. I eventually forfeited.

Having a custom functions with dependencies on external packages such as ajv and built-ins like assert and osdoesn't seem to work out of the box with the current rollup configuration.

Tasks

• Migrate typedEnum as a custom oas function
• Build a very rich custom function, with plenty of dependencies and ensure rollup and karma can digest it without further tweaking
• Ensure the custom function creation documentation is in line with the required steps to make that work for the end user.