Add description property to wireit script configs

[RodrigoHamuy]

Fixes #1015.

Summary

Adds an optional description: string field to wireit script configurations, allowing developers to document what each script does.

• JSON Schema updated with markdownDescription for IDE autocomplete/hover support
• Parsing and validation handled in the analyzer
• TypeScript BaseScriptConfig interface updated
• Test coverage added for valid and invalid values

Usage

"wireit": {
  "build": {
    "description": "Compile TypeScript sources to JavaScript",
    "command": "tsc --build",
    "files": ["src/**/*.ts"],
    "output": ["lib/**"]
  },
  "cleanup": {
    "description": "Delete paths that are not version controlled.",
    "command": "rm -rf some/path/* && rm -rf some/other/path/*"
  }
}

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