refactor(runner): add docs to @vitest/runner, remove barrel files

[sheremet-va]

Description

This PR adds JSDOC to the public runner API and removes barrel files from @vitest/runner package

To generate jsdocs, I used Copilot Chat with a prompt for every method:

generate jsdocs for "<method>" with examples

Some jsdocs were updated after the prompt.

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:.