fix(coverage): istanbul-reports to support projectRoot

[AriPerkkio]

Adds support for configuring projectRoot of istanbul-reports. This is something that nyc does as well: https://github.com/istanbuljs/nyc/blob/v15.1.0/index.js#L465. c8 has similar PR open.

The istanbul-reports uses this internally for reporters ['clover', 'cobertura', 'lcovonly']. Many third party tooling use the pure lcov.info when creating their own reports, e.g. SonarQube.

Example

/workspaces/__vitest-monorepo-coverage
├── package.json
└── packages
   ├── a
   |  ├── package.json
   |  └── src
   |     └── index.js
   └── b
      ├── package.json
      ├── src
      |  ├── add-and-double.js
      |  └── add-and-double.test.js
      └── vitest.config.js

// packages/b/vitest.config.js
export default defineConfig({
  test: { coverage: { provider: "istanbul", reporter: 'lcovonly' } }
});

# Run in project root
$ vitest run --coverage -r packages/b

coverage/lcov.info:
Before: SF:packages/b/src/add-and-double.js
After: SF:src/add-and-double.js