Generate bindings in xtasks, not in the build script

This is inspired by https://github.com/extendr/extendr/pull/611. I think this repository should also have xtasks.

There are mainly two motivations to separate the bindings generation from the compilation:

1. make the compilation of less troubles like https://github.com/extendr/extendr/issues/580
2. target wasm (I'm not sure if this is even possible, but I think it's a matter of time until not a few package authors and users wish the Rust-powered package on webR)