initialized extension with basic functionality

vendor/highlightjs/languages/plaintext.min.js

@@ -0,0 +1,4 @@
+/*! `plaintext` grammar compiled for Highlight.js 11.11.1 */
+(()=>{var t=(()=>{"use strict";return t=>({name:"Plain text",
+aliases:["text","txt"],disableAutodetect:!0})})()
+;hljs.registerLanguage("plaintext",t)})();