move the tailscale web client out of the cmd/tailscale/cli package, into
a new client/web package. The remaining cli/web.go file is still
responsible for parsing CLI flags and such, and then calls into
client/web. This will allow the web client to be hooked into from other
contexts (for example, from a tsnet server), and provide a dedicated
space to add more functionality to this client.
Updates tailscale/corp#13775
Signed-off-by: Will Norris <will@tailscale.com>
Will Norris