Consider nullable annotations in explicit nulls

The @NotNull and @NonNullable type annotations are currently used by explicit nulls to not nullify (or add flexible type to) a reference type from Java signatures.

In addition to these annotations, I suggest we also consider the @Nullable annotation. It indicates a value can indeed be null. In this case, we will always nullify the type (adding | Null), instead of adding a flexible type.

// in Java
@Nullable String f(String s)

// in Scala with explicit nulls currently
def f(s: (String)?): (String)?

// in Scala with explicit nulls considering the annotation
def f(s: (String)?): String | Null

A list of @Nullable annotations we may want to consider:

javax.annotation.Nullable
org.jetbrains.annotations.Nullable
org.jspecify.annotations.Nullable
...