Refactor npm package and add Worker support

[billti]

This PR cleans up the npm package so nearly all code is shared between node.js and the browser, with just a few lines in platform specific entry points. The platform specific entry points are now handled with 'conditional exports' in the package.json, so either Node.js or browser source consuming the package should just "import { ... } from 'qsharp'".

This PR also adds support for Worker threads, and moves the playground to use WebWorkers by default (getting all the wasm work such as compiling and simulating off the main UX thread). As this works similarly in both browser and Node.js, you can see in some of the unit tests added under ./npm/tests how this is used. (It's mostly the same as non-worker usage, just with one-line difference in initialization and clean-up.

As part of getting the above working, I also added configurable logging support. See ./npm/src/log.ts for details on that. (It should be pretty easy to wire this up to both VS Code "output channels" and macros from the Rust 'log' crate also).

I also made a lot of the TypeScript code more 'strongly typed', and it deals with a lot of string parsing and message handling in the 'npm' package now, avoid the need for consumers to do so.

Running the katas works, but I need to clean up the integration in the playground still for some of the refactoring before they are showing in the playground again (just commented out for now). I'll do that as part of a bigger playground UX clean-up.

I touched nearly every line of code in the npm package, so probably just easier to look at the code overall there, rather than the diff. Sorry :-/

[swernli]

Signing off for the one change under compiler (addition of todo). Additionally, this PR could be a good place to move the compiler/qsc_wasm folder to a top-level wasm folder.