Add yarn.lock to version control?

[Johennes]

In #168 (comment) we discussed whether or not to add yarn's lock file to version control. Apparently there is different guidance on this online. yarnpkg/yarn#838 (comment) advises against it whereas the yarn (classic) docs say the opposite. Adding the lock file would enable reproducible tests but might have downsides that we should figure out.

[jhugman]

Looking at these links (and this one) I think I'm convinced we should do this.