ICU-23481 MF2: get standard functions and allow chaining - #4118

The proposed API is minimal, one extra method in MFFunctionRegistry:

public class MFFunctionRegistry {
   ...
   public static MFFunctionRegistry getStandardFunctionsRegistry() { ... }
}

The most interesting part of the PR is the StandardFunctionsChainingTest, showing off what can be done with it.

Checklist

• Required: Issue filed: ICU-23481
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-NNNNN Fix xyz"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-NNNNN 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
• Approver: Feel free to merge on my behalf