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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