Include the Application Services `autofill` component in Firefox Desktop and configure uniffi-bindgen-gecko-js to generate its JS bindings (the components' Cargo.toml, config.toml, lib.rs and moz.build). The remaining changes are the build artefacts from `./mach uniffi generate`: the new RustAutofill.sys.mjs and its API docs, plus the regenerated sibling Rust*.sys.mjs bindings and GeneratedScaffolding.cpp. Adding a component renumbers the shared UniFFI function table, so the existing bindings and scaffolding are regenerated in lockstep. Differential Revision: https://phabricator.services.mozilla.com/D309353