feat: support enable coverage via --coverage

[9aoy]

Summary

support enable coverage via --coverage.

$ npx rstest --coverage

----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
----------|---------|----------|---------|---------|-------------------
All files |     100 |      100 |     100 |     100 |
 index.ts |     100 |      100 |     100 |     100 |
----------|---------|----------|---------|---------|-------------------

Related Links

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[Copilot]

Pull Request Overview

This PR adds support for enabling code coverage collection via the --coverage CLI flag, allowing users to quickly enable coverage without modifying configuration files.

• Adds --coverage CLI option to enable code coverage collection
• Updates documentation to reflect the new CLI option with usage examples
• Adds test coverage for the new CLI functionality

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/core/src/cli/commands.ts	Adds the --coverage CLI option definition
packages/core/src/cli/init.ts	Implements CLI coverage option handling and config resolution
e2e/test-coverage/index.test.ts	Adds test case for CLI coverage flag functionality
e2e/test-coverage/fixtures/rstest.enable.config.ts	Creates dedicated config file for coverage-enabled tests
e2e/test-coverage/fixtures/rstest.config.ts	Removes coverage config to isolate CLI testing
website/docs/zh/guide/start/features.mdx	Adds Chinese documentation for code coverage feature
website/docs/zh/guide/basic/cli.mdx	Documents --coverage CLI option in Chinese
website/docs/zh/config/test/coverage.mdx	Updates Chinese coverage documentation with CLI examples
website/docs/en/guide/start/features.mdx	Adds English documentation for code coverage feature
website/docs/en/guide/basic/cli.mdx	Documents --coverage CLI option in English
website/docs/en/config/test/coverage.mdx	Updates English coverage documentation with CLI examples

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