Install ESLint and prettier with yarn 4 through corepack (closes #493)

yarn 1, which the node image ships and which is no longer developed,
printed node's url.parse() deprecation warning during the js-deps
install. package.json now pins yarn 4.18.1 by version and hash in its
packageManager field; the js-deps stage enables it with the node
image's own corepack and runs `yarn install --immutable`. yarn.lock is
regenerated in yarn 4's format from the old lockfile, so every package
keeps the version it had. The new .yarnrc.yml keeps the install in
node_modules/, where the lint and Markdown stages run the tools from.

Model: opus-5-5

.yarnrc.yml

@@ -0,0 +1,3 @@
+# Install into node_modules/: the Dockerfile's lint and Markdown stages run
+# ESLint and prettier from node_modules/.bin.
+nodeLinker: node-modules