Can we support @Nonnull and @Nullable annotations in public method return values and parameters?

Can we support @Nonnull and @Nullable annotations in public method return values and parameters?
Adding these can help parseq users write accurate Parseq code more easily by getting help from IDE in null detection wherever applicable.

If agreed, I can take this task and submit a review.

No files or tests are named. Start by reviewing Parseq's public Java API declarations and determine the applicable return values and parameters for @Nonnull and @Nullable annotations; done means the supported public API is annotated consistently so IDEs can detect nullability.

Written by the indexing model from the issue text.