yarn2nix: add easy yarn & nodejs pkg overrides

Projects often require a specific major version of NodeJS, and sometimes
a specific yarn version. Since yarn2nix utilities are accessed from
nixpkgs now, there is no simple way to override versions of nodejs and
yarn without complex solutions like an overlay.

This adds `yarn` and `nodejs` as optional attribute arguments to
`mkYarnModules`, `mkYarnPackage`, and `mkYarnWorkspace`. They default to
the same versions that are currently being used, and the nodejs input to
yarn is overridden so that it will match if only `nodejs` is overridden
by the user.

These arguments will also cascade from `mkYarnWorkspace` ->
`mkYarnPackage` -> `mkYarnModules`, making per-package overrides very
simple.

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