skip vite prebundling step

[Rich-Harris]

So this was a fun debugging experience:

Vite scans your app, starting at your HTML files, looking for dependencies to prebundle before it bundles your actual app. Not totally sure I understand why this is necessary in SSR mode, or if build.lib is specified, but it happens.

Unfortunately, it does that by globbing for HTML files in the project directory. In a standard Vite project that's fine, because it excludes the build directory from the globbing, but it obviously doesn't know not to look at a SvelteKit adapter's output directory.

Suppose you're building an app that uses adapter-static to bake out some HTML for every page in your site. Let's further postulate that the site in question has a page for every county in the US, plus a few other pages, and needs to be built twice (the second time for AMP). Finally, imagine that the src/app.html template and its AMP counterpart contain hundreds of elements before you add your content.

Vite will try to parse and walk all ~7,000 large pages at once. There is no --max-old-space-size that would make this possible.

Luckily, disabling it is very easy, hence this PR.