Allow type `T1?|T2?`

[Masquue]

T1?|T2? is now parsed as T1?, whose correct form should be (T1|T2)? or T1|T2|nil or field? T1|T2.

But many libraries (e.g. neovim plugins) are using this incorrect form.

This misuse is so prevalent that should we consider making this form legal?