Improve handling of Vite configuration about fastify-vite

[galvez]

Oh, this issue is tracking something that was done a long time ago.

Just for clarity, when I said config.js, I meant the internal config.js module.

@fastify/vite requires the presence of a vite.config.{ts,js,cjs,mjs} module — even though not technically required, that's an opinionated measure to ensure Fastify and Vite configurations stay separate, leading to better maintainability and preserving the independence of the Vite application from Fastify.

[devmattrick]

config.js seems like it could conflict with unrelated config files. I think it'd be better to scope it how other tools like sveltekit (sveltekit.config.js) and Astro (astro.config.js) do it by just changing the part before the .config.js. fastify-vite.config.js would be a better choice here imo, even though it's a bit wordy.

Also it's worth noting that users may want to define their configs with other extensions, notably .cjs and .mjs. Potentially TypeScript extensions too, but I'm not sure how/ if TypeScript configs are supported.