feat: add assertion helper to hide internal stack traces

[hi-ogawa]

Description

Resolves #4494

This PR implements vi.defineHelper, which works like this:

import { expect, vi } from 'vitest'

const assertPair = vi.defineHelper((a, b) => {
  expect(a).toEqual(b)  // 🙅‍♂️ error code block will NOT point to here
})

test('example', () => {
  assertPair('left', 'right')  // 🙆 but point to here
})

Example output:

FAIL  example.test.ts > example
AssertionError: expected 'left' to deeply equal 'right'

Expected: "right"
Received: "left"

 ❯ example.test.ts:8:3
      7| test('example', () => {
      8|   assertPair('left', 'right')
       |   ^
      9| })

The implementation relies on error stack error filtering based on special marker function __VITEST_HELPER__. This processing happens during parseStacktrace, so the filtering also applies to --printConsoleTrace (which I just realized and maybe we can tweak the behavior.)

The error stack and __VITEST_HELPER__ filtering requires the original error to have proper trace. It turns out Webkit is a bit behind other runtimes, so currently "async" helper doesn't work. But, it looks like it's coming soon as Bun has it already WebKit/WebKit#57832 and this is also reached main line webkit oven-sh/bun#22517. We'll likely just need to wait for a few weeks until Playwright webkit update and see what happens.

TODO

• test
  • console
• doc

TBD

• should printConsoleTrace also hide helper internal?
• rename to vi.defineHelper

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[hi-ogawa]

The API is renamed to vi.defineHelper.

[sheremet-va]

LGMT!