Make non-top level declarations of `typedef` valid

[purplenoodlesoop]

Currently, it is not possible to create a typedef not as a top-level entity. However, they could be useful as a class member (static or not), or in a function body.

It is open to discussing how they should behave in classes, but it looks like it should be entirely possible to do the following:

void foo<T extends Object?>() {
  typedef Double = T Function(T value);
  ...
}

[lrhn]

Existing issue in the SDK repo, which has not been moved to the language repo yet: #2952

[purplenoodlesoop]

Thanks! Closing the issue then – searched in the language repo initially :)

[lrhn]

Seems that another issue, #67, is also in the language repo, with a name that made it unsearchable. My bad 😁.