Add new rule for public domain license

[alok1304]

Fixes #3997

Tasks

• Reviewed contribution guidelines
• PR is descriptively titled 📑 and links the original issue above 🔗
• Tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR
  Run tests locally to check for errors.
• Commits are in uniquely-named feature branch and has no merge conflicts 📁
• Updated documentation pages (if applicable)
• Updated CHANGELOG.rst (if applicable)

Signed-off-by: Alok Kumar [email protected]

[alok1304]

Now, we detects the correct license expression.

"files": [
    {
      "path": "pom.xml",
      "type": "file",
      "detected_license_expression": "public-domain",
      "detected_license_expression_spdx": "LicenseRef-scancode-public-domain",
      "license_detections": [
        {
          "license_expression": "public-domain",
          "license_expression_spdx": "LicenseRef-scancode-public-domain",
          "matches": [
            {
              "license_expression": "public-domain",
              "license_expression_spdx": "LicenseRef-scancode-public-domain",
              "from_file": "pom.xml",
              "start_line": 36,
              "end_line": 42,
              "matcher": "2-aho",
              "score": 100.0,
              "matched_length": 22,
              "match_coverage": 100.0,
              "rule_relevance": 100,
              "rule_identifier": "public-domain_537.RULE",
              "rule_url": "https://github.com/nexB/scancode-toolkit/tree/develop/src/licensedcode/data/rules/public-domain_537.RULE",
              "matched_text": "    <licenses>\r\n        <license>\r\n            <name>Public Domain</name>\r\n            <url>https://github.com/stleary/JSON-java/blob/master/LICENSE</url>\r\n            <distribution>repo</distribution>\r\n        </license>\r\n    </licenses>",
              "matched_text_diagnostics": "licenses>\r\n        <license>\r\n            <name>Public Domain</name>\r\n            <url>https://github.com/stleary/JSON-java/blob/master/LICENSE</url>\r\n            <distribution>repo</distribution>\r\n        </license>\r\n    </licenses>"
            }
          ],
          "identifier": "public_domain-208c1387-6950-7abe-1990-a436120d6de1"
        }
      ],
      "license_clues": [],
      "percentage_of_license_text": 4.2,
      "scan_errors": []
    }
  ]

@AyanSinhaMahapatra can you review this PR, simply I add a new rule for public-domain.