docs: add Profiling Test Performance guide

[AriPerkkio]

Description

Contains two separate commits. Should be merged without squash.

• fix(forks): --cpu-prof and --heap-prof not working
  • Adds work-around from node:child_process.fork does not generate cpu-prof when process is killed nodejs/node#55094 when diagnostic exec args are detected
• docs: add Profiling Test Performance guide
  • Adds documentation about profiling Vitest's test runner and the main thread
  • https://deploy-preview-6555--vitest-dev.netlify.app/guide/profiling-test-performance.html

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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[AriPerkkio]

This should be merged without squash to get fix(forks): --cpu-prof and --head-prof not working visible in changelogs as well.

[hi-ogawa]

Wow, so comprehensive! I tested a profile example and it's very nice!

[sheremet-va]

This looks awesome! Thank you!

[AriPerkkio]

This should be merged without squash to get fix(forks): --cpu-prof and --head-prof not working visible in changelogs as well.

When release is done, let's manually add the fix commit in the changelogs. 🙃