Add @MethodsReturnNonnullByDefault

[Zabuzard]

Similar to #541 , we should take the next step and make our methods also return @Nonnull by default. Fortunately, this is fairly simple with a custom annotation:

@Nonnull
@TypeQualifierDefault({ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
public @interface MethodsReturnNonnullByDefault {
}

(credits go to Minecraft where I snacked this annotation from - kudos to Quinteger#4169 who raised awareness)

The annotation then goes into all package-info.java just like seen in #541 . In the same wave, we should go through the code base and eliminate all explicit @Nonnull, hurray 🙌

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If there is an actual dependency introducing this type already, we should use it instead of "reinventing the wheel".