Previously, we were guessing whether something is optional from the
comment (looking for stuff like "defaults to" or "optional").
To remove this error-prone technique, every optional property or
argument (but not option) is explicitly marked with "?".
As a result, fix documentation issues:
- Removed `TestStep.data` that is not a thing.
- Added `TestConfig.name` documentation.
- Refined a lot of optional types in the documentation.
- Aligned `test.fail()` and `test.slow()` docs with `test.skip()`.
- Strict set of docs vs types inconsistensies in the generator
