Modify Date/Time check in the NumberFormatter for decimal/fractional times

[MarkBaker]

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

Issue #3412

Modify Date/Time check in the NumberFormatter so that masks with a decimal/fractional time aren't misinterpreted as number masks, while still ensuring that durations are interpreted as date/time masks, and locale currency masks are still treated as numbers

A date/time with a decimal time shouldn't have a digit placeholder before the decimal point, only after