wasm-terminal: Fix multi-character font support (ligatures)

[torch2424]

This merges these issues:

https://github.com/wasmerio/webassembly.sh/issues/62

https://github.com/wasmerio/webassembly.sh/issues/49

With the general response:

So this seems to work in Hyper. And to my understanding, they use the xterm-addon-ligatures, which fixes the way the fonts are rendered. However, I linked to the requirements, and this addon needs system access to use the native font renderer. So we are going to half to figure out how to implement this in the browser 🤔

So I went ahead and asked if we'd get browser support in: https://github.com/xtermjs/xterm.js/issues/958 , and can investigate other solutions later on 😄