Add --implements option to scope _Impl scaffold emission

[Copilot]

• Investigate current --implement handling in windows-bindgen
• Add implements: Vec<String> field on Bindgen, --implements CLI arg, and .implements() builder method
• Add Implements matcher type analogous to Filter
• Wire Implements into Config (with should_implement(type, default) helper)
• Gate _Impl scaffolding in interface.rs and cpp_interface.rs based on the new option
• Update docs (lib.rs arg table)
• Extend the harness (fixture.toml keys: implement, implements) and add a bindgen/implements fixture
• Collapse implements and implements_iter into a single implements<I, S>(I) taking an iterator
• Run cargo fmt to fix cpp_interface.rs indentation that broke the fmt CI job
• All 178 fixture tests pass and cargo fmt --check is clean