fix(types): import Mock type from vitest (#43)

The `Mock` type is exported by `vitest`, so importing from `@vitest/spy`
is unnecessary. Also, when installed with pnpm, the `@vitest/spy`
package is inaccessible to the type definitions of this package, because
`@vitest/spy` is not a dependency of it.

Author: aleclarson

src/index.ts

@@ -1,5 +1,4 @@
-import { vi as vitest } from 'vitest';
-import type { Mock } from '@vitest/spy';
+import { vi as vitest, type Mock } from 'vitest';
 
 declare global {
   // eslint-disable-next-line no-var