WIP: wasm32 support

[segeljakt]

In response to murarth/linefeed#67, this is a skeleton for supporting the xterm-js browser terminal. Here is also an example of the xterm-js terminal running (without mortal): https://segeljakt.github.io/arc-script/.

I think some concrete steps to implementing this are:

• Select the functions which we need from the xterm-js API
• Implement bindings for those functions through wasm-bindgen.
• Integrate the bindings with the Mortal API.
• Create a minimal example which puts everything together with wasm-pack

I think I need some feedback on the first and third bullet. The others I can probably solve myself.

[segeljakt]

I created these bindings for xterm-js:

• https://github.com/segeljakt/xterm-js-rs