chore: add funding manifest URL (#17827)

Related to https://github.com/sveltejs/svelte.dev/pull/1632

This will allow us to apply for a grant from floss.fund

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- [ ] Prefix your PR title with `feat:`, `fix:`, `chore:`, or `docs:`.
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### Tests and linting

- [ ] Run the tests with `pnpm test` and lint the project with `pnpm
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.well-known/funding-manifest-urls

@@ -0,0 +1,2 @@
+https://svelte.dev/funding.json
+