tsnet: add opt-in SSH support (Server.ListenSSH)

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This adds tsnet.Server.ListenSSH which, if the SSH feature is linked,
returns a net.Listener whose Accept yields *tailssh.Session values (as
net.Conn). This lets tsnet apps accept incoming SSH connections to
implement custom TUI applications.

Basic apps can use net.Conn directly (Read/Write/Close). Rich apps
import ssh/tailssh and type-assert for peer identity, PTY, signals,
etc. If feature/ssh isn't imported, ListenSSH returns an error.

Includes a demo guess-the-number game in tsnet/example/ssh-game.

Updates tailscale/corp#37839

Change-Id: I4e7c3c96afb030cdf4da8f2d8b2253820628129a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

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