Build a web-based emulator for the documentation site (#1597)

Historically we've used copy-cat to provide a web-based emulator for
running example code on our documentation site. However, copy-cat is
often out-of-date with CC:T, which means example snippets fail when you
try to run them!

This commit vendors in copy-cat (or rather an updated version of it)
into CC:T itself, allowing us to ensure the emulator is always in sync
with the mod.

While the ARCHITECTURE.md documentation goes into a little bit more
detail here, the general implementation is as follows

 - In project/src/main we implement the core of the emulator. This
   includes a basic reimplementation of some of CC's classes to work on
   the web (mostly the HTTP API and ComputerThread), and some additional
   code to expose the computers to Javascript.

 - This is all then compiled to Javascript using [TeaVM][1] (we actually
   use a [personal fork of it][2] as there's a couple of changes I've
   not upstreamed yet).

 - The Javascript side then pulls in the these compiled classes (and
   the CC ROM) and hooks them up to [cc-web-term][3] to display the
   actual computer.

 - As we're no longer pulling in copy-cat, we can simplify our bundling
   system a little - we now just compile to ESM modules directly.

[1]: https://github.com/konsoletyper/teavm
[2]: https://github.com/SquidDev/teavm/tree/squid-patches
[3]: https://github.com/squiddev-cc/cc-web-term

doc/events/file_transfer.md

@@ -29,7 +29,7 @@ for _, file in ipairs(files.getFiles()) do
   local size = file.seek("end")
   file.seek("set", 0)
 
-  print(file.getName() .. " " .. file.getSize())
+  print(file.getName() .. " " .. size)
 end
 ```