Implement missing functions in Interpreter

The interpreter doesn't implement all Power Apps functions yet.

See list of power apps functions: https://docs.microsoft.com/en-us/power-apps/maker/canvas-apps/formula-reference

List of functions implemented in Interpreter is at: https://github.com/microsoft/Power-Fx/blob/main/src/libraries/Microsoft.PowerFx.Interpreter/Functions/Library.cs

Looking at history for that file is an easy way to spot when new functions were added.

Here's are some good previous examples:

Writing test cases
For each function, there should be an individual .txt test case file.
Check expected behavior against Power Apps -with error handling enabled.

Good test cases for the functions include:

• success cases
• failure cases (like passing out-of-range values)
• Passing Blank() or Error values

Start with the Power Apps formula reference and src/libraries/Microsoft.PowerFx.Interpreter/Functions/Library.cs, using its history and the linked examples to select a missing function. Add an individual .txt test case file for the function, covering success, failure, Blank(), and Error values with expected Power Apps behavior. Done means the function is implemented and its tests pass.