fix: change Vite config type to work with Vitest options

[teemingc]

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.

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

[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)

[teemingc]

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.