Use defaultdict on suppress_warning to simplify empty lists

[tupaschoal]

This has two main benefits:

• makes the file a lot simpler since we don't need to define the empty lists for languages without suppressions
• new languages won't automatically fail the CI until someone merges an empty list into the master branch

Will help the follow-up of #3432

Without this patch, adding a new language and running translation_check.py would yield:

PS C:\GitHub\Localisation> python .github/workflows/translation_check.py --reference-lang-file ..\OpenRCT2\data\language\en-GB.txt --master-dir .\data\language --branch-dir .\data\language\
Traceback (most recent call last):
  File "C:\GitHub\Localisation\.github\workflows\translation_check.py", line 245, in <module>
    run()
  File "C:\GitHub\Localisation\.github\workflows\translation_check.py", line 195, in run
    prepare_translation_report(args.master_dir, args.branch_dir, args.reference_lang_file, args.github_run_url)
  File "C:\GitHub\Localisation\.github\workflows\translation_check.py", line 200, in prepare_translation_report
    master_branch = count_translations(master_dir, False, reference_file)
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\GitHub\Localisation\.github\workflows\translation_check.py", line 138, in count_translations
    if base_string in KEYS_TO_IGNORE and base_string not in SUPPRESS_WARNING[lang]:
                                                            ~~~~~~~~~~~~~~~~^^^^^^
KeyError: 'hi-IN'