feat!: `spy.mockReset` changes

[Lordfirespeed]

Description

[!WARNING] This is a breaking change.

See #6378, #6308

This PR changes the behaviour of mockReset to the proposed behaviour of mockRevert in #6378.

That is, spy.mockReset() will reset the inner implementation of spy to its original.

Previously, spy.mockReset would reset the inner implementation of spy to () => undefined.

PR Checklist

• [x] It's really useful if your PR references an issue where it is discussed ahead of time. If the feature is substantial or introduces breaking changes without a discussion, PR might be closed.
• [x] Ideally, include a test that fails without this PR but passes with it.
• [x] Please, don't make changes to pnpm-lock.yaml unless you introduce a new test example.

Tests

• [x] Run the tests with pnpm test:ci.

Documentation

• [x] If you introduce new functionality, document it. You can run documentation with pnpm run docs command.

Changesets

• [x] Changes in changelog are generated from PR name. Please, make sure that it explains your changes in an understandable manner. Please, prefix changeset messages with feat:, fix:, perf:, docs:, or chore:.

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