chore: fix navigation type duplicated descriptions

[teemingc]

closes #15470

This PR ensures that the type descriptions for the various navigation types show up correctly in the IDE when hovering over the type. This results in slightly worse docs when viewing the OnNavigate and AfterNavigate sections but the trade-off is worth it. We get the correct descriptions after type-narrowing too.

The thing about TypeScript and unions is that it tries to merge the description for similar properties. If you have something like:

type A = {
  /** foo */
  example: 'foo';
}

type B = {
  /** bar */
  example: 'bar';
}

type C = A | B;

The description for C['example'] would be "foo bar".

However, if you have some overlap, TypeScript merges your descriptions as best as it can:

type A = {
  /**
  * This contains:
  * - foo
  */
  example: 'foo';
}

type B = {
  /**
  * This contains:
  * - bar
  */
  example: 'bar';
}

type C = A | B;

You'll end up with:

This contains:
- foo
- bar

which works nicely for us in our case.

---

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Not sure if this needs a changeset since the user isn't really affected... Maybe it does? Might even be a breaking change? The types should still be equivalent though.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-15490-svelte.vercel.app/

[teemingc]

This had to be split into two types. Otherwise, their descriptions wouldn't be separable for AfterNavigate and OnNavigate which both exclude the 'leave' navigation type.

[teemingc]

The period helps separate this sentence from the next one it'll be concatenated with https://github.com/sveltejs/kit/pull/15490/changes#diff-9e558fcdee2759e3a180281c10a134e7caa6123e746f91178e20ec1b9d61be76R1364