feat(debug): add debug utility

[mcous]

This PR adds the ability to debug a vitest-when mock:

import { when, debug } from 'vitest-when'

const coolFunc = vi.fn().mockName('coolFunc')

when(coolFunc).calledWith(1, 2, 3).thenReturn(123)
when(coolFunc).calledWith(4, 5, 6).thenThrow(new Error('oh no'))

const result = coolFunc(1, 2, 4)

debug(coolFunc)
// `coolFunc()` has:
// * 2 stubbings with 0 calls
//   * Called 0 times: `(1, 2, 3) => 123`
//   * Called 0 times: `(4, 5, 6) => { throw [Error: oh no] }`
// * 1 unmatched call
//   * `(1, 2, 4)`

The debug function logs by default, but also returns an object that can be used by other tooling:

const debugInfo = debug(coolFunc, { log: false })
console.dir(debugInfo, { depth: 3 })
// {
//   name: 'coolFunc',
//   stubbings: [
//     {
//       args: [ 4, 5, 6 ],
//       behavior: {
//         type: 'throw',
//         error: Error: oh no
//             at /Users/mc/projects/vitest-when/test/vitest-when.test.ts:357:62
//       },
//       calls: []
//     },
//     {
//       args: [ 1, 2, 3 ],
//       behavior: { type: 'return', value: 123 },
//       calls: []
//     }
//   ],
//   unmatchedCalls: [ [ 1, 2, 4 ] ],
//   description: '`coolFunc()` has...'
// }

[mcous]

@BeniRupp I've been giving some thought to #10, and I wonder if something like this debug interface, combined with a user-side mock registry, could be used to implement the feature you were after

[BeniRupp]

@mcous, that looks awesome! I think this debug function can be very helpful. Thanks for pushing that topic! 👏