Adding a standalone calculator extension

[robotfishe]

Made this extension for my own numberpad build. It adds a basic 4-function calculator that displays the entered equation and result on an attached display, and includes a macro button that will output the result to the connected computer.

[xs5871]

I'm genuinely conflicted about this.
As a coding challenge or exploratory blog post, it'd be really interesting -- although the actual implementation would still need some serious work.
As a core part of the firmware that already struggles to fit on certain MCUs, I'm going to question the added value.
If it would enable using the CP REPL on a keyboard display, which is a super-set of the proposed functionality, I'd say that's dumb enough to have some merit. (It'd also make the entire logic ten times simpler.)