[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/vec3.ts

@@ -65,7 +65,7 @@ test3(obj?:Test):Test|null {
 }
 
 static getFullyQualifiedName():string {
-  return 'MyGame_Example_Vec3';
+  return 'MyGame.Example.Vec3';
 }
 
 static sizeOf():number {