v2.0.0-dev.20

• Please refer to https://fzyzcjy.github.io/flutter_rust_bridge/guides/miscellaneous/whats-new for what's changed in V2.
• Fix module parsing when two modules have the same name #1664 (thanks @alanlzhang)
• Fix integrate command's default template has rust_lib hardcoded instead of custom names #1669
• Support Vec<Vec<u8>> for CST/DCO codec (SSE codec already supports it) #1663
• Make LLVM dependency optional #1666
• Add checks of versions #1667