Extend "Custom stores" example & tutorial

[TorstenDittmann]

Extended custom store example for passing data to a method.

When I started working with svelte, an example like the one in this PR would have helped me a lot.

related: #3053

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Tests

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[Rich-Harris]

Thank you, though I'm going to close this — the added example is perhaps a bit confusing (it reimplements set with a different name in a tutorial that's about not exposing set).

While people do trip up over passing parameters to functions (the difference between on:click={foo} and on:click={() => foo(42)}), that's not specific to custom stores, and is shown elsewhere in the tutorial (e.g. two chapters later, and there's a chapter on inline event handlers). I think we need to be careful not to fall into the trap of explaining everything in every tutorial chapter, because it makes the tutorial as a whole longer and more repetitive. It's designed to be read from start to finish, rather than for each chapter to be completely standalone.