Allow loading settings from JSON?

Saw this on twitter: https://twitter.com/hardfist_1/status/1397332063330521090

> really wish emscripten supports json configure like tsconfig.json，writing all configuration in a script is awful

I think it's a really good idea to allow specifying at least common settings in a JSON file instead of a long list of shell parameters, and would make configuration easier.

cc @sbc100 as you've been working on loading specific settings from newline-delimited files, and this seems like a higher-level extension of the same idea