Update ts codegen (#8421)

Makes the return type of `static getFullyQualifiedName()` be a string literal instead of just the string type

Update tests

Co-authored-by: Björn Harrtell <bjornharrtell@users.noreply.github.com>

tests/ts/union_vector/book-reader.ts

@@ -24,7 +24,7 @@ mutate_books_read(value:number):boolean {
   return true;
 }
 
-static getFullyQualifiedName():string {
+static getFullyQualifiedName(): "BookReader" {
   return 'BookReader';
 }