Prompting wasm32-unknown-emscripten users to install Emscripten

Describe the problem you are trying to solve

The wasm32-unknown-emscripten target is unique (AFAIK) in that it requires an external toolchain, Emscripten, to be additionally installed to work correctly. Further, different versions of Rust require different specific versions of Emscripten to be installed and activated to ensure compatibility between the versions of LLVM used by Rust and Emscripten. Unfortunately, the matching Rust and Emscripten versions are not really documented anywhere. While we plan to improve the documentation situation, we will still have the problem of surfacing this issue and its documentation to users who install the wasm32-unknown-emscripten component.

Describe the solution you'd like

I could imagine Rustup helping in multiple ways when wasm32-uknown-emscripten is installed, from simply printing a generic message pointing to documentation, to printing a message identifying the specific version of Emscripten to install, to actually installing the correct version of Emscripten automatically. For now, I would be satisfied if we could just print a generic message. Is that something Rustup would accept a PR for? Would it make sense to pursue doing more than just printing a generic message in the future?

cc @RReverser and @kripken

The issue concerns the wasm32-unknown-emscripten component and prompting users about the required Emscripten toolchain. Begin by locating where rustup handles component installation and user-facing messages; done means installing this component produces a generic message pointing users to the relevant documentation.