Improve typing example

[zrcoder]

I have seen this wonderful example: Typing text on the screen, but still needs some improvement：

• show cursor(this is done in this example: https://github.com/faiface/pixel-examples/tree/master/typewriter)
• move the cursor when the mouse button or arrow keys clicked
• delete character when pressed backspace or delete key
• show line numbers, just like a code input field, like below:

[zrcoder]

I found games of Codemonkey, there is a code text field on the right half of screen, like below:

Codemonkey's games are written in js and html, maybe easy to support such a widget, can Pixel support one?