JS: thread.js requires h$fds and h$fdReady to be declared for static code analysis, minimal

code copied from GHCJS (fixes #24602)

I've just copied some old pieces of GHCJS from publicly available sources (See https://github.com/Taneb/shims/blob/a6dd0202dcdb86ad63201495b8b5d9763483eb35/src/io.js#L607).
Also I didn't put details to h$fds. I took minimal and left only its object initialization: `var h$fds = {};`