fix: correctly detect internal links when hash router is used

[eltigerchino]

fixes #13287

This PR fixes the client-side external link evaluation so that when using the hash router, accessing /index.html is considered an internal link. This prevents the page from reloading infinitely because the router incorrectly considered /index.html an external link.

There's also a slight doc change. Instead of suggesting all links start with /#/, suggesting links start with # allows the router to preserve the /index.html pathname. For example, links to /about should be #/about instead of /#/about so that it navigates to /index.html#about.

---

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

Edits

• Please ensure that 'Allow edits from maintainers' is checked. PRs without this option may be closed.

[changeset-bot]

🦋 Changeset detected

Latest commit: 45c5159

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[Rich-Harris]

preview: https://svelte-dev-git-preview-kit-13296-svelte.vercel.app/

this is an automated message

[eltigerchino]

Seems to still infinitely reload if we embed the app in an iframe using srcdoc because the URL becomes about:srcdoc and the pathname is srcdoc.