Ability to prerender to `about.html` instead of `about/index.html` like Sapper

Describe the bug When setting tralilingSlash: 'ignore' then the static adapter generates wrong files, e.g.

• blog/hello-world.html.svelte is generated as blog/hello-world.html/index.html
• about.svelte is generated as about/index.html

To Reproduce Here is a repo with issue reproduction, generated files are also pushed to the main branch in dist directory: https://github.com/bdadam/svelte-kit-static-issue

The repo was freshly created with npm init svelte@next ..

Expected behavior

• blog/hello-world.html.svelte => blog/hello-world.html.html
• about.svelte is generated as about.html

Information about your SvelteKit Installation:

• The repo was freshly created with npm init svelte@next .
• yarn add -D @sveltejs/adapter-node@next to install the static adapter

Severity For me this is a blocker in adopting Svelte Kit for one side project. I have a website which has 6+ years old URLs with .html extensions, e.g. https://example.com/blog/hello-world.html. I don’t want to touch these URLs due to all the SEO rank and backlinks they received throughout the years. In all honesty this is a toy project - but I guess I’m not alone with this requirement.

[Conduitry]

A recent-ish new feature in Sapper was to have foo.html.svelte render to foo.html when exporting a static site (rather than foo.html/index.html), but apparently that didn’t make its way into SvelteKit. I think that would be a reasonable enhancement.

In any case, about.svelte should never generate about.html, as they correspond to different routes (/about vs /about.html).

[bfanger]

I’ve rebased PR 2632, any feedback that prevents merging this PR would be appreciated.