Convert lib/types dir to TypeScript

[owenpearson]

Resolves #790

[QuintinWillison]

In an ideal world I would have liked to see new API commentary alongside at least some of the modules, types and methods/functions. This could be an opportunity to inject some of that, though I recognise it may not have been a priority for you.

I'm also noticing a lot of variance in terms of indentation. e.g. common/lib/types/stats.ts vs common/lib/util/dataSizeBytes.ts. I've not pulled the code in locally to inspect further but the impression is that some files are using tabs and others using spaces. I assume, therefore, that we're not running any tools over the code in CI that would spot for formatting inconsistencies? I think we should be and these tools should form foundational underpinnings to this work - otherwise we're only going to need to go back over all the files again some time in the future to conform them.

[owenpearson]

@QuintinWillison The tabs vs spaces stuff should be fixed now.

As for the API commentary that would go on the external type interface (ably.d.ts) and would be unrelated to this work. Maybe one day we can automatically generate the external type interface from the source code but it would be hard to do that without exposing a bunch of internal typings that we wouldn't want users of the library to see.