Fix #20287: Add flexible types to Quotes library

[noti0na1]

Fix #20287

To fix the missing case, we have to add flexible types to Quotes library.

[noti0na1]

Hi @nicolasstucki, do you have any idea why the extractor for FlexibleType does not work in Extractors.scala and SourceCode.scala: TypeRepr is not a valid result type of an unapply method of an extractor.

[nicolasstucki]

This should probably be included in 3.5 as it is a part of https://github.com/scala/scala3/pull/18112. Specifically, the changes in the TASTy format require this change in the reflection API.

[sjrd]

The core meeting decided to take this PR in on 3.5.0.