Consider adding nullness annotations

[kressi]

Nullness annotations have been asked for before, #1701 and #1919. Back then, the tooling and standards have not been where they are today. In the meantime JSpecify annotations have been widely accepted by the community. IDEs offer reasonable support and NullAway can verify nullness checks at build time. Therefore, adding nullness annotations will let vavr users make better use of the tooling.

What's the general opinion on adding nullness annotations and running nullness checks as part of the build?

[kressi]

Created #3098 to illustrate how nullness checks could look on Option. With nullness annotations, users will be warned about potential issues with following code.

Option<@Nullable String> optionA = Option.of("").flatMap(__ -> null);

With nullness annotations on Option, the compiler will warn about the lambda with 'null' is returned by the method declared as @NullMarked.