[TS] Fix getFullyQualifiedName codegen for typescript (#7730)

#7451 caused getFullyQualifiedName to return a name with underscores,
not periods. Because the fully qualified name is a property of
FlatBuffers, not the language being codegen'd for, it should use
periods. Fixes #7564.

Co-authored-by: Derek Bailey <derekbailey@google.com>

tests/ts/my-game/example/struct-of-structs.ts

@@ -28,7 +28,7 @@ c(obj?:Ability):Ability|null {
 }
 
 static getFullyQualifiedName():string {
-  return 'MyGame_Example_StructOfStructs';
+  return 'MyGame.Example.StructOfStructs';
 }
 
 static sizeOf():number {