Use compiler flags instead of feature for nightly features

[ranile]

See yewstack/yew#2754 for rationale. Tokio also uses the same approach with tokio_unstable feature

This will allow downstream users, who use cfg for nightly features, to enable nightly features of proc-macro-error2) by using build.rs to set the flag). It's currently impossible to do this: yewstack/yew#3752

[Alextopher]

@ranile what do you think about #5 where proc-macro-error2 has the build script that sets the relevant flags?

I've left that PR fall off my plate but all it is missing is CI I can try to push over the line.