[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/test.js

@@ -24,7 +24,7 @@ export class Test {
         return true;
     }
     static getFullyQualifiedName() {
-        return 'MyGame_Example_Test';
+        return 'MyGame.Example.Test';
     }
     static sizeOf() {
         return 4;