`yarn test` not compatible with node 16

[jakub-g]

Just FYI, using node 16.8.0 locally I get this:

jakub ~/dev/typanion(main:-) $ yarn test                     
node:fs:2650
      const dev = BigIntPrototypeToString(stats.dev, 32);
                  ^

TypeError: BigInt.prototype.toString requires that 'this' be a BigInt
    at Number.toString (<anonymous>)
    at gotStat (node:fs:2650:19)
    at /Users/jakub/dev/typanion/.pnp.js:21601:13

(it works fine with 14.x, I'll stick with that)

[arcanis]

Yep, I need to upgrade Yarn on this repo 👍