Clean up signals when Luau reports signal GC

[Sleitnick]

Previous behavior: Signals remained partially alive until the attached GameObject was destroyed. This is essentially a mem leak, as the resource is no longer needed but is staying around.

const conn = button.onClick.Connect(() => {
    conn.Disconnect();
    // 'onClick' is created and now exists for the remainder of 'button' gameObject's
    // lifetime even though 'conn' is disconnected. This includes both resources in Luau and in C#.
});

New behavior: Luau reports to C# when a Luau signal is GC'd, thus allowing C# to clean up C#-based signal resources once no longer needed. There are also various fixes on the Luau side that address signals being pinned longer than they should.

const conn = button.onClick.Connect(() => {
    conn.Disconnect();
    // 'onClick' can be safely GC'd on the Luau side (no connections active). C# resources are cleaned up.
});

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No change required from anyone. These changes are opaque in nature, mostly on the Luau plugin side.