Merge Istanbul coverage reports into a single report.

Outputs:

All fields are required.

• source: Path to the source folder (usually ${{ github.workspace }})
• coverage-reports: Path to coverage reports (usually ${{ github.workspace }}/coverage)
• output-folder: Where to output merged reports
• formats: Comma-separated list of report formats (ie., json,lcov)
  • Options include cobertura, clover, json, lcov, teamcity, text-loc, text. text-summary and json-summary are always included.

• summary: Coverage summary
• json: JSON coverage summary