v0.1.3 of udpipe_flutter

Flutter library and demo app for UDPipe 1 — NLP tokenization, POS tagging and dependency parsing across 50+ languages, with native FFI on desktop/mobile and WebAssembly on the web. Changelog excerpt: - Fixed pub.dev documentation score: resolved dartdoc library-name conflict, excluded demo-app files from public API, added missing dartdoc comments. - Fixed pub.dev static analysis score: added assets/models/README.mdso the declared asset directory exists in the published package. - assets/models/README.mddocuments where to download UDPipe model files.