Fix race condition in webworker logic (hopefully)

[DanielSherlock]

Aims to fix issue #17.

Makes use of existing variable workerWorking to keep track of when worker is busy.
This starts off true, because the worker script takes a while to register it's own onmessage event handler (it has to wait for the gleam compiler to be available).
When the worker is ready, it sends a message with initialReadyMessage: true. If the worker hasn't yet compiled the initial code, this will cause it to do so.

Little Caveat: This version of the code relies on the editor's onUpdate event listener to send the initial message to the worker. If this turns out not to be reliable (see big caveat below), we can reinstate the line at the end which manually sends the first message.

Big Caveat: This code has so far only been tested using devtools. Having devtools open does change the behaviour of the original code (it reliably caused the otherwise-intermittent issue), so it may equally change the behaviour of this version of the code too. Ideally I would want to test it once the project has been built properly with gleam run, served to localhost, and probably with some network delay or whatever too. That's why this PR is a draft at the moment.

I've not got a working install of node on my machine (this is a target: javascript project), so testing this change may take me a little while, sorry.

[lpil]

Is this ready for review? Thanks!

[lpil]

This looks fab so I'm gunna merge! 😃