Add implementation to TextProcessorJDK.distinctLetters() for haiku-kata-solutions module

[rzrobin213]

The method distinctLetters() in the TextProcessorJDK class for the haiku-kata-solutions module is not yet implemented. Follow the TODOs and hints given in the method. Please do not add solutions to the non-solution modules. After writing the code, uncomment the test and tag annotations and check that the test case distinctLetters() in the TextProcessorJDKTest passes for the haiku-kata-solutions module. Make sure to adhere to our contribution guide.

Please note that this issue is created for GHC 2023 and 2024 Open Source Day, thus priority will be given to the attendees of this event. After the event, this issue will be open to anyone.

[esterjingtw]

I am GHC'23 attendee. I am interested to pick up this issue.