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/hand-fan.ts

@@ -40,7 +40,7 @@ mutate_length(value:number):boolean {
   return true;
 }
 
-static getFullyQualifiedName():string {
+static getFullyQualifiedName(): "HandFan" {
   return 'HandFan';
 }