Add WASM support

[swenson]

Moved some functions and constants around so common ones are in ui_common.js.

ui_js.go Mostly contains a basic call out to prompt() to ask for input. It's not perfect, but it works and is simple.

[dekimsey]

Hi, not a maintainer. But I've run into this myself with WASM targets. This is great, thank you for taking the time to make it work! And test! :swoon:

I think my only ⛏️ is would you mind including a copy of the error that set you down this path? I think it's a common problem with terminal libraries. Would be nice to have something reference.

[swenson]

The errors in this repo were:

# github.com/bgentry/speakeasy
../../go/pkg/mod/github.com/bgentry/[email protected]/speakeasy.go:23:18: undefined: getPassword
# github.com/fatih/color
../../go/pkg/mod/github.com/fatih/[email protected]/color.go:21:12: undefined: isatty.IsTerminal

Usually these missing functions are because of underspecified build tags, like using go:build !windows and assuming it is a UNIX os that will have access to the UNIX syscall, or just not having a definition specified for go:build js or go:build wasm.

[swenson]

Closing in favor of hashicorp#1