Fix text parser to use codeblocks

This means that JavaScript and JSON tiddlers will be properly
highlighted if the highlight plugin is loaded.

core/modules/parsers/textparser.js

@@ -15,11 +15,11 @@ The plain text parser processes blocks of source text into a degenerate parse tr
 var TextParser = function(type,text,options) {
 	this.tree = [{
 		type: "element",
-		tag: "pre",
-		children: [{
-			type: "text",
-			text: text
-		}]
+		tag: "$codeblock",
+		attributes: {
+			code: {type: "string", value: text},
+			language: {type: "string", value: type}
+		}
 	}];
 };