Support natspec in event/state variable/public state variable

[kanej]

Similar to #328, provide a subset of natspec auto-inserts on a /** */ completion before an event, state variable or public state variable.

This should be based on the docs: https://docs.soliditylang.org/en/latest/natspec-format.html#tags.

For events it seems to @dev and @param.

But we should investigate why public state variables have an @return while state variables don't.