Add global type registry

[sessfeld]

When messages are imported, add them to a global type registry as a side-effect. That allows users to omit typeRegistry when interacting with Any.

I'm adding this a draft right now, because this definitely requires changes to the docs, and probably another test case. I'm also not sure about the typecast I'm using in jsonReadOptions and jsonWriteOptions. I originally did this implementation as a patch to the compiled package, so there were no types.

Closes #131

[timruffles]

Are you still working on this, and is there anything I can do to help? Would be very useful.

[sessfeld]

Full disclosure: I haven't touched this PR since I opened it, but we've been using this implementation internally since then.

Where I was unsure is with the typecast that I had to do after the mergeJsonOptions, I have no idea if this is correct like this.

[timostamm]

Apologies for the late reply. I have to focus efforts on moving away from the TS compiler API for code gen, so that we can update to a more recent version of TS. It's a big effort, and side-effects in imports are controversial enough that I prefer to punt on them for now.