Add warnings for inferred flexible types in public methods and fields

[noti0na1]

Report warning if a public method/field exposes FlexibleType in its result type under explicit nulls and encourage explicit annotation.

According to the discussion in #23682

def f(s: String) = s.trim // warn

-- Warning: Stest.scala:100:4 --------------------------------------------------
100 |def f(s: String) = s.trim // warn
    |    ^
    |Public method f exposes a flexible type (String)? in its inferred signature. Consider annotating the type as String or String | Null explicitly
1 warning found

[som-snytt]

OT it would be interesting to infer the type (nullability) of an override from the RHS for an effectively final member. Then some explicit types would not be necessary when implementing Java interfaces.