We are only using `references` in a solution-style tsconfig.
According to discussions at microsoft/TypeScript#60465,
such usage doesn't require `composite: true` to be set in sub-configs.

Removing this field loosens the constraints on these configs that all
files to be explicitly listed in `files` or `includes`.

After this change, type errors in source code would only be reported
twice if they're also imported by unit test specs, in contrast to
always be reported twice prior to the change.
I know this is not ideal yet, but it's still an improvement, and might
help catch some edge cases such as #437 (comment)

In the long run, we should still keep an eye on #549
(pending vuejs/language-tools#4750).
Cross-referencing might be a more intuitive configuration, and should be
the desirable configuration if we opt into Vitest Browser Mode.