Add some tui features

[oovm]

I want to interactively select prompt words in wasi (using up, down, and enter keys), but it seems impossible.

I also failed to compile crossterm with wasm32-wasip2 as the target.

I hope that in the next stage, some basic tui capabilities can be added to the terminal interface.

Similar to is-tty, ansi detection, terminal windows info.

[sunfishcode]

Yes, that's roughly the idea of the terminal interface.

I made a rough draft of a possible API here:

https://github.com/sunfishcode/wasi-cli/blob/77e2126752d0febec82f20ad5c9ab3560d571e55/wit/terminal.wit

One thing that might be a little surprising about it is that it takes the approach of defining a set of escape codes, instead of, say, allowing the codes to just be terminal-specific and exposing something like a "TERM" environment variable and requiring applications to use something like terminfo or termcap. I'd be curious if anyone has thoughts about that.

[oovm]

LGMT, different platforms use different methods, such as $TERM, $HOST, mode con, these are platform differences, and need to be decoupled at the wasi level.

Stepping back, if users want some specialized capabilities, they can still obtain them through environment variables.

[sunfishcode]

I've now tidied this up and posted a draft PR at #62.

Stepping back, if users want some specialized capabilities, they can still obtain them through environment variables.

It's an interesting question whether we should permit unrecognized escape sequences to pass through. It would let applications access specialized capabilities, but at the same time it'd create portability hazards. My guess at this time is that it'll be better to disallow this, and say that if there are features applications want that not all terminals support, we can add them as optional features, similar to how full-screen mode is an optional feature in the current proposal.

[oovm]

I think it should be banned, it will cause platform differences and make wasi less cross-platform.