Closes #4910.

This replaces the transformPage API with transformPageChunk, so that we can respond in a streaming fashion in future. We're not yet taking advantage of that ability, but updating the API is the first step.

Previously, transformPage took the entire HTML document:

export function handle({ event, resolve }) {
  return resolve(event, {
    transformPage: ({ html }) => html.replace('old', 'new')
  });
}

Now, it takes a chunk of HTML plus a done boolean. Chunks are not guaranteed to be well-formed HTML (they might include the opening tag of an element but not its closing tag, for example) but nor are they arbitrary — they will always be split at sensible boundaries (like %sveltekit.[property]% or before the contents of a layout or page component). Because of that, many transforms will work identically:

export function handle({ event, resolve }) {
  return resolve(event, {
-    transformPage: ({ html }) => html.replace('old', 'new')
+    transformPageChunk: ({ html }) => html.replace('old', 'new')
  });
}

In some cases, it's necessary to operate on the entire document in one go — for example, to validate and transform an AMP document. In these situations it's easy to buffer the HTML and return the transformed document at the end:

export function handle({ event, resolve }) {
  let buffer = '';
  return resolve(event, {
    transformPageChunk: ({ html, done }) => {
      buffer += html;
      if (done) return amp.transform(html);
    }
  });
}

Right now, done will always be true (i.e. there'll only be one chunk, which is the entire document) but that will change in the near future.

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