Use binding_abi::flag for SpellIsTargeting and SpellCanTargetUnit so addons
can compare their results with 1, as in 1.12.1. Keep nil for false.
Verified on Windows with Rust 1.98.1: three regression failures before
the fix, all 35 focused spell tests passing after it, formatting and
render-pass lint clean. The full UI run passed 1677 tests, with one
ignored and one traceback-name failure reproduced on untouched main.
Fixes#711