Improve console UI

This one would be pretty easy for somebody to help with.

The current console UI kind of sucks: it's just a giant `div` that we stuff text in. We should try to use a terminal emulator.
- text will wrap properly
- we can autoscroll to the bottom better
- _probably_ won't lag during an ios build, when tons of text gets logged all at once (slowness might be due to other stuff)

This task is a prerequisite for being able to type back into the console (currently is read-only), and being able to search console output.


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How to improve it?

Consider using https://github.com/sourcelair/xterm.js, the actively maintained fork of termjs.

This should fix text-wrapping, scrolling, and _probably_ performance (although we may also want to batch text output into the console). It will set us up to be able to run commands via the console and search console output.

Start by locating the current console UI and the code that feeds text into its giant div, then review the proposed xterm.js integration. Done means console text wraps correctly, scrolling reaches the bottom reliably, and output performance is acceptable during an iOS build; consider batching output if needed.

Written by the indexing model from the issue text.