support composer based extensions

[WolfgangFahl]

examples:

{
  "extensions": [{
    "composer": "{ \"require\": { \"mediawiki\/semantic-result-formats\": \"~3.1\" } }",
    "name": "Semantic Result Formats",
    "purpose": "Additional result formats for Semantic MediaWiki queries",
    "localSettings": "wfLoadExtension( 'SemanticResultFormats' );",
    "url": "https://www.semantic-mediawiki.org/wiki/Extension:Semantic_Result_Formats"
  }, {
    "name": "Page Forms",
    "composer": " \"mediawiki\/page-forms\": \"^5.2.1\"",
    "localSettings": "wfLoadExtension( 'PageForms' );",
    "purpose": "(known before 2016 as Semantic Forms) is an extension to MediaWiki that allows users to add, edit and query data using forms",
    "url": "https://www.mediawiki.org/wiki/Extension:Page_Forms"
  }]
}