Add style to distinguish button disabled state

[AsafAgranat]

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Tests

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[Rich-Harris]

Thank you! I agree with this change, but I reckon it probably makes sense for it to be built in to the default stylesheet, rather than specific to this example — so I've updated sveltejs/template instead, which also updates the tutorials (or will when I redeploy the site)

[AsafAgranat]

I thought exactly that on the next instance I encountered a disabled button in the tutorial, but was too engrossed in the tutorials to do something about it :)