Add VBlank interrupt lesson to Unbricked

[KingParmenides]

Closes #157.

Summary

• add a new Part II lesson covering VBlank interrupt vectors, handlers, register preservation, rIE/rIF, ei, reti, and halt
• add a buildable unbricked/vblank-interrupts stage
• replace the later Unbricked frame-loop LY polling with a VBlank interrupt-backed WaitForVBlank helper

Testing

• PATH=/private/tmp/rgbds-v1.0.0:$PATH ./build_all.sh from unbricked/
• PATH=/private/tmp/mdbook-v0.4.52:$PATH CARGO_HOME=/private/tmp/cargo-home CARGO_TARGET_DIR=/private/tmp/gb-asm-tutorial-target mdbook build
• CARGO_HOME=/private/tmp/cargo-home CARGO_TARGET_DIR=/private/tmp/gb-asm-tutorial-target cargo test --locked
• git diff --check