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/my-game/example2/monster.ts

@@ -24,7 +24,7 @@ static getSizePrefixedRootAsMonster(bb:flatbuffers.ByteBuffer, obj?:Monster):Mon
   return (obj || new Monster()).__init(bb.readInt32(bb.position()) + bb.position(), bb);
 }
 
-static getFullyQualifiedName():string {
+static getFullyQualifiedName(): "MyGame.Example2.Monster" {
   return 'MyGame.Example2.Monster';
 }