Proposal: Switch the project package manager from NPM to Yarn

[DuncanBetts]

Justification: It would resolve the problem this PR tries to resolve: Pull 580 - that one of the dependencies currently depends on a dependency that is a git submodule, and that NPM doesn't handle this gracefully.

That PR doesn't entirely solve the problem, it's just a partial workaround, whereas switching to Yarn would solve the problem (which is upstream).

That PR (and also #579) could then be dropped unmerged.