Feature: Web REPL using Emscripten

[MathematicalDessert]

Closes #177

Based on @zeux's comment , I have started implementing a Web REPL for Luau.

This PR will have the initial base for the REPL (potentially including site design).

[MathematicalDessert]

I've implemented a small web demo for Luau using Emscripten. The design is nothing special and I'm sure more visually talented people will make it look fancy.
Future:

• Same thread execution (toggle?)
• Syntax highlighting for input + rich output
• Better looking site design
• Error stack trace (?)

How to use:

• Build using Emscripten compiler (I used cmake: emcmake cmake <luau directory>, cmake --build <cmake config output dir>)

• Run webserver in output directory (e.g., python 3 http.server`)

[MathematicalDessert]

With a little more work I've managed to add the demo to the existing docs:

[zeux]

Thanks! Left some comments.

[zeux]

Thanks! Going to merge this a little later today or tomorrow.