Made Connection property `.Connected` public & other Signal changes

[lucasmz-dev]

This PR has made these changes:

• Connections now have a proper .Connected property. Why, GoodSignal?
• :DisconnectAll actually disconnects all connections, .Connected is set to false when it is called.
• Added some extra type declaration on Signal methods, not big of a change;

The :DisconnectAll change means that the signal class is more consistent with BindableEvents and RBXScriptSignal behaviour as a whole.

For example, this piece of code changes what connections are fired depending on whether you're using my modified code or not. The modified version is consistent with what happens with a normal RBXScriptSignal.

local Event = Signal.new()

Event:Connect(function()
    -- Fired second

    print("This will not fire with the new changes")
end)

Event:Connect(function()
    -- Fired first

    Event:DisconnectAll() -- Same as :Destroy on instances
    -- On RBXScriptSignals, and on the modified version, these connections are tagged as not connected, once :Fire goes over the next node (next connection), it will see that it's not connected, and not run it.
end)

Event:Fire()

Note: This PR has not been tested.

[jaipack17]

Looks great to me! Will merge this before the next release. Thanks for the contribution!