missing docs: web worker

Start with the WebWorker section in README and review the existing worker examples and caveats. Document the messages the worker accepts and returns, its limitations, and whether files from node_modules/sql.js/dist can be used; also cover the missing @types/sql.js worker definitions. Done means users have a complete WebWorker usage explanation on the website or GitHub wiki.

The README says

If you don't want to run CPU-intensive SQL queries in your main application thread, you can use the more limited WebWorker API.

It gives a couple examples and caveats, but I need a lot more information to use the feature. I need a list of messages I can pass, what the worker will pass back, and a description of what the limitations are. There's no page on the website or GitHub wiki about this.

Other issues:

• The docs say to get the file from the releases page, but I'd rather get them from node_modules/sql.js/dist so they get version updates. But someone in this issue said that won't work. This is something that should be covered in a full explanation of how to use web workers.
• @types/sql.js is missing types for the web worker API.
• Existing issue caused by these missing docs: https://github.com/sql-js/sql.js/issues/490

Because of these missing docs, for my app I'm going to write my own custom web worker using the normal SQL.js API.

I'll also highlight the proposal to remove the whole web worker API, which would be a way to avoid this whole issue (though you'd still want the docs to have some hints about how/why to write a custom web worker): https://github.com/sql-js/sql.js/issues/377