Adds an on-demand GitHub Action that publishes the package to the npm
registry (currently under tailscale-connect, will be moved to
@tailscale/connect once we get control of the npm org).
Makes the package.json for the NPM package be dynamically generated to
have the current Tailscale client version.
Updates #5415
Signed-off-by: Mihai Parparita <mihai@tailscale.com>
