Adding support for constant denominators

• Introduced a new field for constant denominators in MeasureUnitImpl.
• Added methods to set and retrieve constant denominators in MeasureUnit.
• Updated parsing logic to handle unit constants correctly.
• Enhanced unit tests to cover new functionality, including constant denominator scenarios.
• Improved code readability and consistency across MeasureUnit and SingleUnitImpl classes.

TODO in the following PRs

1. Add support to unit constant during units conversion.
2. Add support to unit constant during units formatting.

Checklist

• Required: Issue filed: ICU-22781
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable