Explore AssemblyScript integration

We have a PR https://github.com/playcanvas/engine/pull/2213 which explores one way of using AssemblyScript (WASM) modules by Playcanvas. I would like to explore a way to build custom functions using AssemblyScript, which can be called by the main engine in JS to optimize hot functions / but perhaps do more in the future.

Main objectives:

• build system should build the engine as currently, but build wasm module along it from separate ts files.

  perhaps https://www.npmjs.com/package/rollup-plugin-assemblyscript could be useful here

  but also this is interesting as it allows to embed the wasm module into main script
  https://blog.scottlogic.com/2017/10/30/migrating-d3-force-layout-to-webassembly.html
  https://github.com/rollup/plugins/tree/master/packages/wasm

• AssemblyScript allows code to be compiled to WASM, but also generate TS code. It would be good to have this as an option during development / debugging, allowing easy debugging of AssemblyScript code in form of TS.

Later:

• AssemblyScript has support for SIMD and atomics (experimental features), it'd be great to see if these can be used. I'm not sure what the impact is of the code using these still being able to compile to TS.