New LocalizerDiagnosticMode and fixed MissingTranslation

[ErikApption]

• The JsonStringLocalizer class has been updated to use the LocalizerDiagnosticMode property. If LocalizerDiagnosticMode is true, the JsonStringLocalizer will return a LocalizedString where the value is a string of repeated 'X' characters.
• The JsonStringLocalizer class has been updated to write missing translations to a file for each locale. Previously, all missing translations were written to a single file.
• The MissingTranslationsTest class has been updated to check for a file for each locale when testing if missing translations are tracked.
• A new test class DiagnosticTranslationTests has been added to test the new LocalizerDiagnosticMode functionality.

[ErikApption]

@AskmethatFR Hey Alex, would you be able to look at this one?