Convert library code to TypeScript

[owenpearson]

Resolves #686

• Converts the library code to TypeScript. I've also opted to rewrite the code using ES6+ syntax which is compiled down to ES3 using the TypeScript compiler.
• I've used a few generally undesirable TypeScript features here in the interest of being able to get this change landed sooner (in particular I've used the any type quite widely and there are quite a few explicit type casts). We don't need to keep a note of these since they can easily be identified using ESLint and I do intend to fix them in the near future.
• The bundle size for ably.min.js (the minified browser bundle) has increased somewhat (from 193.5kb to 222kb). I don't think there's too much we can do about this right now without a lot of effort/risk, and I think there are better things we can do to reduce bundle size for lib consumers in future (like making the library more compatible with tree shaking).

Note for reviewers: All of the changes in this branch have been reviewed independently so don't feel it necessary to read through all of the changes included here; I'm really just looking for more general feedback on the overall structure of what's being changed here (although feel free to comment on specific library code changes if you want).

[QuintinWillison]

I've pretty much skim read thru, given we know that all of this work has already been reviewed once in upstream PRs.

Just a few things that I think could still have a tidy, or I've only just spotted...

[lmars]

Epic effort! LGTM