Add support for imaginary numbers

Problem Statement
Windows Calculator currently does not support imaginary numbers. Instead, we show "Invalid input":

Evidence or User Insights
The complex number system is part of US high school common core curriculum. According to some reports in the Math Educators Stack Exchange, complex numbers are taught in at least some high school level math and science courses in China, Norway, Italy, India, Israel, and the UK.

Proposal
Add support for imaginary numbers in Scientific mode of Windows Calculator.

Goals

• Add support for output that includes imaginary numbers (e.g., √(-16) = 4 i)
• Add support for inputting expressions with imaginary numbers (e.g., i² = -1)
• Add i to the keyboard

Non-Goals

• Support for imaginary numbers outside of Scientific mode

Low-Fidelity Concept

Requested Assignment
I'm just suggesting this idea. I don't want to implement it.

Start by reviewing the Scientific mode requirements described in this issue, including imaginary-number output, expression input, and the Scientific-mode keyboard. Done means Scientific mode supports examples such as √(-16) = 4i and i² = -1, with an i key, while other modes remain out of scope.