feat: new ssr.fileExtension option

[antfu]

Allow meta frameworks to explicitly set the desired file extension. For example, in VitePress heavily relies on the file name conversion by adding .js to .md files for the chunk name. The current solution provides reasonable default, but may not be stable enough for meta-frameworks to consume (may vary on users' package.json).

Alternative

Or maybe, we could always generate as .js with a fake package.json to force the type resolution.

[patak-dev]

We could already use entryFileNames to override the default. And you can keep the name but fix the extension with it, no? It is hidden in build.rollupOptions.output.entryFileNames but this is a more general issue. I would like if we provide a default build: { entryFileNames, chunkFileNames, assetsFileNames } as shortcuts to these, that would be generally useful

[bluwy]

I'm also slightly leaning towards build.rollupOptions.output.entryFileNames to prevent config overlap. Maybe we could document instead of Vite's extension default for cases like this.

[antfu]

I am doing this because of vuejs/vitepress#707

In VitePress it assumes the output is .js (https://github.com/vuejs/vitepress/blob/52499221874e23ec437c416f4fc5ef3edb771857/src/node/build/render.ts#L52), where with the previous #8382, the output will not be stable when the users set type: module for their project root.

[antfu]

Alright, end up using https://github.com/vuejs/vitepress/blob/e0f30159b3141e48bfd3a4cc277880cdf486fabe/src/node/build/bundle.ts#L76-L80. Close for now to minimalize the config inteface.