Prerendering bails if files already exist

[Rich-Harris]

Describe the bug
If you build an app with adapter-static without first clearing out the build directory, the prerenderer will bail if it encounters pages that were built in a previous run. This is bad.

Severity
Bad

Additional context
The issue stems from this code...

kit/packages/kit/src/core/adapt/prerender.js

Lines 189 to 200 in 59bc85d

	const file = parsed.pathname.replace(config.kit.paths.assets, '').slice(1);
	const file_exists =
	(file.startsWith(`${config.kit.appDir}/`) && existsSync(`${dir}/client/${file}`)) ||
	existsSync(`${out}/${file}`) ||
	existsSync(`${config.kit.files.assets}/${file}`) ||
	existsSync(`${config.kit.files.assets}/${file}/index.html`);
	if (file_exists) {
	seen_files.add(resolved);
	continue;
	}

...which was necessary at one point because we needed to bail out of rendering if a static file corresponded to the request, and didn't have a better mechanism. Since we now generate a manifest of the contents of static, I'm pretty sure we can just use that (though we would also need to filter out the contents of appDir or make the prerenderer aware of those files individually as well)