Fix parser to handle multi-line JSON format and add local testing

This commit fixes the parser to handle both JSON lines and multi-line JSON
formats from govulncheck, and adds tools for local testing:

- Updated parser to detect and handle both JSON lines and pretty-printed JSON
- Added logic to skip govulncheck installation if already present
- Created run-local.js for testing the action locally without GitHub Actions
- Added test:local npm script for easy local testing
- Changed example to use a version of golang.org/x/net with known vulnerabilities

The parser now correctly identifies 14 vulnerabilities in the example,
including ones that trace to the html.Parse call in main.go.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

package.json

@@ -7,7 +7,8 @@
     "build": "ncc build index.js -o dist",
     "test": "jest",
     "test:watch": "jest --watch",
-    "test:coverage": "jest --coverage"
+    "test:coverage": "jest --coverage",
+    "test:local": "node test-local.js"
   },
   "keywords": [
     "github",