fix: force string conversion in prism

It looks like there is a difference in behavior that I don't really get, but without forcing the string conversion, all our tests are failing with "`code.trim` is not a function" because `code` contains an object and not a string (it is the result of the `require` calls to laod the dynamic snippets). I may be missing something obvious, but this fixes the issue.

projects/demo/src/app/prism.service.ts

@@ -8,6 +8,6 @@ import 'prismjs/components/prism-typescript';
 export class PrismService {
 
   highlight(code: string, lang: string) {
-    return prism.highlight(code.trim(), prism.languages[lang], lang);
+    return prism.highlight(code.toString().trim(), prism.languages[lang], lang);
   }
 }