Fix SignalMethods.connect callback return type

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SignalMethods.connect typed its callback return as boolean | undefined. That rejects the most common handler shape — one that returns nothing — because a void-returning function is not assignable to boolean | undefined:

events.connect('changed', () => {
    doSomething(); // returns void -> Type 'void' is not assignable to 'boolean | undefined'
});

Switching to boolean | void accepts both a void-returning handler and one that returns true to stop emission, which matches the imports.signals runtime semantics (only true stops emission — see _signals.js).

Fixed here for now; the file already carries a TODO to upstream SignalMethods into @girs/gjs, where the same type lives — that's a separate change in the types generator.

Validated locally (build:types, prettier:check, validate:types, validate:example) — all green.

Closes #116

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These changes were merged prematurely by mistake; main has been rewound to before the merge so the change can go through review first. Superseded by #133 (same branch/commits).