Fixes compatibility with `mergeConfig`, as reported in vuejs/create-vue#313

Before the change, `defineConfig` would return `UserConfigExport`, which
is a union type. This would cause `mergeConfig` to fail, as it doesn't
accept the `UserConfigFn` type in the union. (vitejs#13135)

After the change, `defineConfig` returns the same type that it receives,
so it would exclude `UserConfigFn` from the return type whenever
possible.

Based on vitejs#13792 but with slightly more verbose function style
configuration types.
We can make the generic version pass the tests with some hack, but it
is too permissive that it would allow users to pass in unknown fields.
The overload version is much more direct and simpler.

Thanks @xiaoxiangmoe for helping me debug this.