chore: make site work when base path is configured

[benmccann]

We've got a number of base path issues I've been looking at. It's helpful to be able to run the site with a base path so that I have something to debug against. I don't expect that we'll necessarily maintain kit.svelte.dev in a perfectly working state with base path, but it's helpful to have something nonetheless

I found one serious bug while working on this and fixed it in another PR (#8683)

This also bumps vite-imagetools which just fixed base path support in its latest release, so it was a good test of that as well

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[Rich-Harris]

I found one serious bug while working on this and fixed it as part of this PR

Can we open a separate PR for that, with a test?

[benmccann]

Can we open a separate PR for that, with a test?

Done. I moved that code to #8683 and added a test

[Rich-Harris]

nice