Add module for typesafe messages

[freya022]

Please see the readme for details about this feature.

• Add enum LocaleScope
  • Values:
    • PREFER_USER: Use the user locale if available
    • GUILD: Use the guild locale
• Add @PreferLocaleScope, throw if both a preferred scope and a required locale parameter exists
  • value: LocaleScope
• Support first parameter being DiscordLocale/Locale, null = use initial locale
• Convert camelCase parameters to snake_case
• Throw if a formattable parameter is nullable or the argument is null at runtime (Use Objects.requireNonNull(T, String) to simplify)
• Add FormattableArgumentConverter/ArgumentFormatter, maybe extend on ParameterResolver
  • Examples of built-ins:
    • Do not add Duration unless the user is able to localize those
    • IMentionables where any mentionable entity is transformed into its mention
  • Add @FormatWith to force usage of a specific formatter, for example the default IMentionable formatter could be overridden for Member and instead insert the member's effective name
• (Breaking) Add LocalizableTemplate#arguments: List<FormattableArgument>
• Add FormattableArgument#canFormat(Class<?>)
  • Return true by default, document that in unknown cases, it should return true and just let it fail at runtime
  • Make sure to pass the final type (after conversion)