root: add .nvmrc (#24440)

build: add .nvmrc

The setup docs tell contributors to install Node.js with nvm, but there
was nothing in the repo for it to read, so the version had to be passed
by hand on every `nvm install` / `nvm use`. Add an .nvmrc pinning 24,
which is the floor declared in package.json's `engines.node` and the
version the setup guides name.

Running newer than the pin locally is fine: CI resolves `>=24` through
setup-node's `node-version-file: package.json` and so tracks the latest
satisfying release.

Co-authored-by: Playpen Agent <279763771+playpen-agent@users.noreply.github.com>

.nvmrc

@@ -0,0 +1 @@
+24