fix: always default paths.assets to paths.base

[jdkula]

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This fixes the issue discussed in #8919 where the build process will fail if trailingSlash = 'never' in the root page and a paths.base is set. Essentially, the build process would eat the last element of the base path and subsequently error out.

When trailingSlash = 'never';, when prerendering /$base/ (our root), the prerenderer gets redirected to '/$base' at prerender.js:305 (which strikes me as invalid, it's kind of "before the root"). Then, that will cause us to visit /$base, which will return the HTML we want rendered at the /$base. This is a problem though, because at this point all relative links we crawl will be resolved relative to /$base, so when we call resolve at prerender.js:250 with, for example, ./favicon.png, we'll end up with a call like resolve('/$base', './favicon.png'), which will ofc resolve to /favicon.png (and not /$base/favicon.ico, which is what we want).

This was resolved by adding a special case in prerender.js's visit function that, if the current URL matches the config.paths.base url exactly, then it will add a trailing slash (causing relative paths to resolve as children to the root/base directory rather than as siblings).

[dummdidumm]

Thank you! This was probably caused through #8838 (since the issue says it starts erroring on 1.4 or later), but I don't fully understand how. @benmccann @Rich-Harris maybe you can have a closer look as well.

[Rich-Harris]

thank you! the actual fix was a bit different, but the failing test was super helpful for diagnosing it

[benmccann]

my bad. I swear I had the code written as Rich has it now because that's how it was before my change. I must have accidentally lost it at some point. glad we have a test for it now! thank you!