Support configuring coverage reporter options

[AriPerkkio]

Clear and concise description of the problem

Add support for configuring various options of different Istanbul reporters. This applies to both @vitest/coverage-c8 and @vitest/coverage-istanbul.

https://github.com/DefinitelyTyped/DefinitelyTyped/blob/276d95e4304b3670eaf6e8e5a7ea9e265a14e338/types/istanbul-reports/index.d.ts

Reporters have various options that solve different issues, e.g. using relative paths in monorepo setups: #1902 (comment).

Suggested solution

Similar API as Jest has https://jestjs.io/docs/configuration#coveragereporters-arraystring--string-options.

Vitest's documentation could look like this:

reporter

• Type: string | string[] | [string, {}][]
• Default: ['text', 'html', 'clover', 'json']
• Available for providers: 'c8' | 'istanbul'
• CLI: --coverage.reporter=<reporter>, --coverage.reporter=<reporter1> --coverage.reporter=<reporter2>

Coverage reporters to use. See istanbul documentation for detailed list of all reporters. See @types/istanbul-reporter for details about reporter specific options.

The reporter has three different types:

• A single reporter: { reporter: 'html' }
• Multiple reporters without options: { reporter: ['html', 'json'] }
• A single or multiple reporters with reporter options:

  {
    reporter: [
      ['lcov', { 'projectRoot': './src' }],
      ['json', { 'file': 'coverage.json' }],
      ['text']
    ]
  }

Alternative

No response

Additional context

• Requires unreleased c8@7.13.0.
• Requires refactoring Vitest's internal coverage typings first

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.