feat: allow the rendering of relative paths

[steamer2k]

resolves #595
resolves #9569

Allows building SPA sites with a relative asset path.

E.g., whether server from:

• https://some.server/foo
• https://some.server/foo/bar
• https://some.server/foo/bar/baz

...the SPA should be able to find the _app/ subdirectory as a sibling to the fallback page. E.g., given a fallback page of index.html:

someDirectory/
  index.html
  _app/
    immutable/
      assets/...
      chunks/...
      entry/...
      nodes/...
    env.js
    version.json

When kit.paths.relative is set, this change renders references to the _app/ directory with a leading . so the browser interprets them as relative paths.

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[steamer2k]

I'll see about following up on tests, lints and changesets tomorrow.

[Pandoks]

This also allows you to serve the web app through file:// for things like embedded systems, tvs, first class electron support, etc.

Any app where you have direct access to the files and serve start from the index.html without needing to inline your entire app if it's huge.