Play Solo & Test Server auto connect (#840)

When enabled, the `baseurl` of the session is written to
`workspace:SetAttribute("__Rojo_ConnectionUrl")` so that the test server
can connect to that session automatically.

This works for Play Solo and Local Test Server. It is marked
experimental for now (and disabled by default) since connecting during a
playtest session is... not polished. Rojo may overwrite things and cause
headaches. Further work can be done later.

plugin/src/Settings.lua

@@ -14,6 +14,7 @@ local defaultSettings = {
 	twoWaySync = false,
 	showNotifications = true,
 	syncReminder = true,
+	autoConnectPlaytestServer = false,
 	confirmationBehavior = "Initial",
 	largeChangesConfirmationThreshold = 5,
 	playSounds = true,