[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.js

@@ -21,7 +21,7 @@ export class StructOfStructs {
         return (obj || new Ability()).__init(this.bb_pos + 12, this.bb);
     }
     static getFullyQualifiedName() {
-        return 'MyGame_Example_StructOfStructs';
+        return 'MyGame.Example.StructOfStructs';
     }
     static sizeOf() {
         return 20;