feat: add json summary report

[bahmutov]

which generates report in coverage/coverage-summary.json like

{
    "total": {
        "lines": {
            "total": 3,
            "covered": 3,
            "skipped": 0,
            "pct": 100
        },
        "statements": {
            "total": 3,
            "covered": 3,
            "skipped": 0,
            "pct": 100
        },
        "functions": {
            "total": 1,
            "covered": 1,
            "skipped": 0,
            "pct": 100
        },
        "branches": {
            "total": 0,
            "covered": 0,
            "skipped": 0,
            "pct": 100
        }
    },
    "/Users/gleb/git/instrument-example/src/App.js": {
        "lines": {
            "total": 1,
            "covered": 1,
            "skipped": 0,
            "pct": 100
        },
        "functions": {
            "total": 1,
            "covered": 1,
            "skipped": 0,
            "pct": 100
        },
        "statements": {
            "total": 1,
            "covered": 1,
            "skipped": 0,
            "pct": 100
        },
        "branches": {
            "total": 0,
            "covered": 0,
            "skipped": 0,
            "pct": 100
        }
    },
    "/Users/gleb/git/instrument-example/src/index.js": {
        "lines": {
            "total": 2,
            "covered": 2,
            "skipped": 0,
            "pct": 100
        },
        "functions": {
            "total": 0,
            "covered": 0,
            "skipped": 0,
            "pct": 100
        },
        "statements": {
            "total": 2,
            "covered": 2,
            "skipped": 0,
            "pct": 100
        },
        "branches": {
            "total": 0,
            "covered": 0,
            "skipped": 0,
            "pct": 100
        }
    }
}

[bahmutov]

🎉 This PR is included in version 3.7.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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