feat: support experimental doc entries
- Params/options/members are marked as experimental in the docs.
- `experimental.d.ts` is generated that contains all types and
includes experimental features.
- `experimental.d.ts` is references in our tests so that we
can test experimental features.
- `fonts` option is restored as experimental.
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
- Each overload, e.g. for `page.evaluate`, shows a nice autocomplete doc,
not only the first one.
- We can have multiple overloads directly on the docs page, e.g.
`test.skip(title, fn)` and `test.skip(condition, description)`.
These overloads are internally named `Test.skip#1` and all aliased
to `test.skip`.
