fix(spy)!: support spying on classes

[sheremet-va]

Description

Closes #6104

This is a breaking change.

This is not supported anymore:

vi.spyOn(mock, 'HelloWorld').mockImplementationOnce(() => {
  const Mock = vi.fn()
  Mock.prototype.hello = vi.fn(() => 'hello world')
  return new Mock()
})
const hw = new mock.HelloWorld()

But you can replace it with a function:

+ vi.spyOn(mock, 'HelloWorld').mockImplementationOnce(function () {
  const Mock = vi.fn()
  Mock.prototype.hello = vi.fn(() => 'hello world')
  return new Mock()
})
const hw = new mock.HelloWorld()

But ideally you should instead do something like this:

const HelloWorld = vi.spyOn(mock, 'HelloWorld')
HelloWorld.prototype.hello = vi.fn(() => 'hello world')
const hw = new mock.HelloWorld()

Or this (previously not supported):

const HelloWorld = vi.spyOn(mock, 'HelloWorld').mockImplementation(class Mock {
  hello = vi.fn(() => 'hello world')
})

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• 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.
• Ideally, include a test that fails without this PR but passes with it.
• Please, don't make changes to pnpm-lock.yaml unless you introduce a new test example.

Tests

• Run the tests with pnpm test:ci.

Documentation

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

Changesets

• 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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