chore(#787): migrate from yarn to npm

[garethbowen]

Closes #787

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This is quite risky because unfortunately there was no way to convert back to npm while preserving the lock file so it required a clean npm install. I manually reverted some known libraries that a pegged to a version but I may have missed some. So we should regression test this fully before release, BUT moving WF to CF will also be risky and require regression testing so I suggest we defer it until the next minor Central release.

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[latin-panda]

It looks good! I haven't done a deep test yet, but I did leave a couple of minor fixes below.

[garethbowen]

@latin-panda I'd like to get this merged soon because if anything changes in the yarn.lock it's hard to port it to the package-lock.json. However it's fairly risky because of unknown changes in dependencies. However because we've just released now is the perfect time to merge and find any issues. So I'm going to go for it. Sorry in advance if I've missed anything!