Fix: handle data: and about: protocols better

[kran6a]

Related: #13231, #13226

Always consider about: and data: protocols internal urls.
Fix the invalid base error by setting base_expression to new URL('#', location) when using the about: or data: protocol.

Sorry for not adding tests. I don't have experience writing tests on browser environments.
I am currently using this PR locally to build widgets that are embedded on a webapp via an iframe with srcdoc. Widgets load and navigation works, but they are so simple I can't ensure there are no bugs even on common use cases.

Note that when using srcdoc you must use absolute URLs or otherwise the iframe will take the host's URL as the base. This is not svelte-kit-specific behavior.

Feel free to try the PR and/or suggest changes and I will add them to the PR when I have some free time.

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