feat: provide ContextKey type for better typing of setContext/getContext

[dummdidumm]

Wanted to see if this requires a breaking change, which would be an RC blocker, and the only way to find out was to implement it. Draft mode because we need to agree on the detailed API of this PR.

Not completly ideal because you can circumvent the type safety by doing getContext<SomeType>(context_key) - changing this would require a breaking change, which we could do in Svelte 6 after we've given ContextKey some time to establish itself. Also doesn't add the interesting type narrowing idea in https://github.com/KamenKolev/svelte-typed-context/blob/master/index.ts#L14 (yet), probably easier to do together with said breaking change.

closes #8941

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[olehmisar]

Hey. Did you see my attempt #9522? It has a check for hasContext, so this code will be properly typechecked:

const a = getContext(key); // T | undefined
if (hasContext(key)) {
  const b = getContext(key); // T
}

Not sure if this is worth the complexity added, though.