fix: change Vite config type to work with Vitest options

[eltigerchino]

closes #8768

As suggested by Rich and Dominik, this changes all vite.config templates to:

• use defineConfig instead of type annotations.
• import defineConfig from vitest if the project includes vitest.

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[dummdidumm]

Is this a breaking change since it changes the shape of the vite config? If so, we should release it together with the other breaking change (and further breaking changes possibly coming out of the svelte package discussion)

[eltigerchino]

Is this a breaking change since it changes the shape of the vite config?

As in how the config is exported? (changed from exporting the constant to directly exporting the method output).

I'm not sure if it's breaking. It still uses the vite UserConfig type, but gets the type inference from the defineConfig output instead. Only the vitest option has changed to correctly use the superset type from vitest instead of vite.

[dummdidumm]

It's more along the lines of potential downstream projects like svelte-add modifying that code. @babichjacob is vite.config.js/ts something svelte-add touches/adders could touch?

[Rich-Harris]

approved, but will hold off merging till we get clarity around svelte-add

[babichjacob]

Svelte Add is fine with defineConfig - it doesn't even validate that it's from Vite (or even called defineConfig - just that it's a function) so using the one from Vitest is a-ok. Thanks for the heads up, and you can go ahead.