Docs: update react / reactive store guides to use twoslash examples

The docs PRs for the react stack (https://github.com/anza-xyz/kit/pull/1779, https://github.com/anza-xyz/kit/pull/1782) currently use ` ```ts` for their examples. This is not typechecked in the same way as `ts twoslash` and does not support the nice cut before syntax.

This is because docs uses a published Kit version, and we haven't yet published a version with the new code/changes.

When we do, and we update docs to 7.x, we should do a quick sweep and move most of these to `ts twoslash`.

Start with the documentation changes in PRs 1779 and 1782, and check that the published Kit 7.x version is available before beginning. Sweep the react and reactive store guides for the affected TypeScript examples; done means most applicable examples use ts twoslash and support the intended cut-before syntax.

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