jspecify, fixes, optimizations

[magicprinc]

Thank You for an amazing project!

Small changes you could find useful 🤝

1. force project indentation rules (4-8 spaces) in .editorconfig
2. fix .gitignore: all subprojects have "out" sub-directory: they all must be ignored
3. first steps to jspecify: jspecify is the new industry standard for Nullable annotations; Spring 7 uses jspecify
4. optional Dispatcher configuration
5. more javadocs
6. a couple useful utilities from our internal code base
7. code compaction using lombok