TypeScript: Added missing keyword undefined (#2088)

This adds `undefined` to the list of keywords in TypeScript.

components/prism-typescript.js

@@ -1,6 +1,6 @@
 Prism.languages.typescript = Prism.languages.extend('javascript', {
 	// From JavaScript Prism keyword list and TypeScript language spec: https://github.com/Microsoft/TypeScript/blob/master/doc/spec.md#221-reserved-words
-	'keyword': /\b(?:abstract|as|async|await|break|case|catch|class|const|constructor|continue|debugger|declare|default|delete|do|else|enum|export|extends|finally|for|from|function|get|if|implements|import|in|instanceof|interface|is|keyof|let|module|namespace|new|null|of|package|private|protected|public|readonly|return|require|set|static|super|switch|this|throw|try|type|typeof|var|void|while|with|yield)\b/,
+	'keyword': /\b(?:abstract|as|async|await|break|case|catch|class|const|constructor|continue|debugger|declare|default|delete|do|else|enum|export|extends|finally|for|from|function|get|if|implements|import|in|instanceof|interface|is|keyof|let|module|namespace|new|null|of|package|private|protected|public|readonly|return|require|set|static|super|switch|this|throw|try|type|typeof|undefined|var|void|while|with|yield)\b/,
 	'builtin': /\b(?:string|Function|any|number|boolean|Array|symbol|console|Promise|unknown|never)\b/,
 });