feat: expose non-top-level production classes

[saschanaz]

This does not expose top level productions for now, since the parse function of them have some weird internal-only behavior, e.g. Interface.parse does not parse interface keyword by itself. Technically same for Attribute and Operation, but they are usable at least.

This patch includes:

• A relevant test (N/A, just exposes things)
• A relevant documentation update

[saschanaz]

But I guess #702 is blocking you, @marcoscaceres ?

[saschanaz]

Turns out I pushed the wrong button.