Change worker chunk name from <ID> to <ID>.worker

[gluck]

Fixes #43, as it avoids conflicts with other number-based webpack
chunks.
Note that the conflict may not be on the filename (since this plugin
adds '.worker' to it) but on the internal key used by webpack in
referencing modules.

E.g. in the below generated webpack loader code, using w0 avoids conflict with chunkId 0 that would otherwise never load.

I'd argue that a cleaner "fix" would be to name the chunk with something like worker.${id} and drop the chunkFilename replacement logic (that inserts .worker. before the output extension), but that'd break the naming of those who provided a name to the worker, so you tell me.

[developit]

TBH I think I agree with your point about the cleaner fix @gluck - it'll break existing names, but perhaps that's okay? Could even append .worker to the name, then remove .worker.worker as a way to make it backwards-compatible.

I just published 4.0.0 - if we can make a call on this relatively soon, I'd be okay with doing this as a minor 4.1 since nobody has yet upgraded.

[gluck]

Done, I was zealous and included suggested #19 change at the same time (because a user could use this feature to revert/customize the naming behavior), lmk if you think I should remove it.

[gluck]

(note that I couldn't make it backward-compat because the check for duplicate worker.worker. would have to be done after webpack has resolved chunkFilename and I don't know how to hook there).
I could inject into .worker into chunkFilename if it doesn't include [name] to make sure output files always include .worker in them if you think it's a good idea, but I'm personally not sure it is.

[developit]

I think this looks good! I'll need a little bit before I can do a proper thorough review. We'll want to think about whether this constitutes a v5 or not given the naming change.