Make SPA fallback try each directory component

[sdunster]

This PR adds logic to make SPA fallback look for a corresponding index.html for each directory component all the way down to root.

In dev mode if the browser requests /foo/bar we will try looking for an index.html like this:

• /foo/bar/index.html
• /foo/index.html
• /index.html

This way if you're developing a suite of single page applications that use SPA routing you should no longer get not found errors when you refresh/go direct as posted about here.

This is my first contribution - would be great to get some indication of whether or not a PR like this is likely to be accepted before I go writing tests/updating docs - just let me know what is needed.

[bluwy]

The issue with this is that not all (or many) deployment platforms support this behavior. Vite dev currently mimics the most common behavior so that it matches after you've deployed it. Can you confirm which platforms or server setups you use that may benefit from this?