misc/wasm: expect environment to provide polyfills
The list of environments to support with wasm_exec.js was becoming too
large to maintain. With this change, wasm_exec.js expects that the
environment provides all necessary polyfills.
The standardized "globalThis" is used for accessing the environment.
wasm_exec.js now only provides stub fallbacks for globalThis.fs and
globalThis.process.
All code specific to Node.js is now in a separate file.
Change-Id: I076febbd94d4d7845260faad972f450f74a7b983
Reviewed-on: https://go-review.googlesource.com/c/go/+/347353
Trust: Richard Musiol <neelance@gmail.com> Reviewed-by: Cherry Mui <cherryyz@google.com>