[major] Main module is public.

[dtzSiFive]

Clarify main module is implicitly/always public.

Update examples to specify widths (and concrete reset type).

Some of these are invalid regardless (e.g., "input foo : UInt" on main module) but recent CIRCT checks this now while parsing.

[dtzSiFive]

• [X] : Make "public" keyword required on main modules.
• [X] : Update examples accordingly.