Add Doxygen `\memberof` annotations

[amomchilov]

trafficstars

Doxygen's \memberof command lets you associate C functions to types, to make them render as if they were actual methods. This is in a similar spirit to \extends, which we use to simulate inheritance from pm_node.

Demo:

It's possible to hide the PRISM_EXPORTED_FUNCTION macro spam from these return types, but I'll do that in a separate PR.

[amomchilov]

@kddnewton Ready for re-review

[kddnewton]

Superseded by https://github.com/ruby/prism/pull/3641