add `wasm32-unknown-emscripten` to the conda-forge build of rust

[DerThorsten]

The rust build from conda-forge is missing the wasm32-unknown-emscripten-target. Therefore we cannot use the package as it is for emscripten-forge and need to maintain our own recipe.

Potential difficulties /problem:

• (some?) python packages with rust extensions required rust-nighty to be used over non nightly builds, but that
  might not be relevant.
• Are all rust compilers working with any emscripten version we are using?

[anutosh491]

I guess this would suiffce ?
conda-forge/rust-feedstock#213

Can get rid of the R recipe if that's the next step here