Proposal to split linking part of emcc.py into emlink.py

Today emcc.py act as both the compiler driver and the linker. In the interests of separation of concerns and in order to make emscripten more componentized I've been working on a change to put all the linking logic in emlink.py and have emcc.py call this if needed under the hood.

This change should also allow for other use cases, such as taking an externally built wasm file (from rust, or go, or kotlin, or wasi) and running emlink.py on it in order to "emscriptenize" it and create JS and html around it.