Adds getNextMockImplementation

Description

Adds a method to get the mock implementation including implementations, which are temporary and created with mock...Once. Fixes https://github.com/vitest-dev/vitest/issues/7030.

A new method was added in order to remain backwards compatible. If this is not desired, it can be replaced.

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We don't need a new method for this. The old one should return the current implementation

@sheremet-va Sorry for the delay. I have removed the getNextMockImplementation() and have instead overwritten the getMockImplementation() logic.