feat: fallback to original mockImplementation if no match

[mcous]

Overview

As discussed in #15 (comment), this PR makes a slight adjustment to the mockImplementation added by vitest-when. Rather than returning undefined in the case of a miss, it will use whatever mockImplementation already existed on the spy prior to when usage.

This will keep the default undefined return in normal usage, but allow for the configuration of unconditional default implementations, which may be helpful in specific cases.

Closes #15

Change log

• Fall back to original getMockImplemenation if a call misses all configured stubbings
• Add additional type annotations and docs I miseed in fix(types): accept MockInstance as stubbing target #16