Add dt baseline folder to gitignore (#20205)

Author: weswigham

.gitignore

@@ -67,3 +67,4 @@ tests/cases/user/*/**/*.d.ts
 !tests/cases/user/zone.js/
 !tests/cases/user/bignumber.js/
 !tests/cases/user/discord.js/
+tests/baselines/reference/dt