Adds a tool/yarn helper script that uses specific versions of yarn and
node, downloading them if necessary.
Modeled after tool/go (and the yarn and node Redo scripts from the
corp repo).
Also allows the path to yarn to be overidden (in case the user does not
want to use this script) and always pipes yarn output (to make debugging
and viewing of process easier).
Signed-off-by: Mihai Parparita <mihai@tailscale.com>
Mihai Parparita