Add Wasm backend
The Wasm backend can just use LLVM which I think can emit Wasm. The pure Fortran part of the runtime library should translate. The C part of the runtime library might translate with Clang, we need to investigate.
We need to add the proper option to enable Wasm into the main lfortran
driver, and make sure everything works. Add a demo for something simple.
I have asked about some of these details here: https://github.com/pyodide/pyodide/issues/549#issuecomment-926915889