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

[olhotak]

In the case of a method, should we also warn if the parameter types are flexible? Or can that never happen for some reason?

[noti0na1]

I don't think it's possible to infer parameter types.

[olhotak]

Otherwise this LGTM, but let's wait for the CI.