Add simple key tester

[fauxpark]

Description

Some mildly interesting notes on Windows:

• The VK value alone is not reliable enough as it changes based on the keyboard layout language. Thankfully, a PS/2 equivalent scancode is also provided, presumably for backward compatibility, or just vestigial
• Each KeyControl has to be set as disabled, as they apparently grab focus and prevent the key events from being intercepted by the form. Unfortunately this also changes the legend label text colour, and doesn't seem to be (easily) overrideable

Some mildly interesting notes on macOS:

• Caps Lock only generates flagsChanged events, as a result when caps lock is on the key is considered pressed.
• The Help (Insert) key doesn't generate a keyDown event
• There is no VK enum entry for the Application/Context menu key, but it is recognised
• F20 is recognised and has a VK enum entry, but all the Apple keyboards I could find on Google Images didn't have it, and there's not really a good place to put it
• F13-F15 are mapped to the equivalent keys on a "normal" keyboard layout (Print Screen, Scroll Lock, Pause/Break) although I'm not sure if F14 and F15 can ever be pressed as they seem to just change the brightness on my MBP
• The media keys don't appear to generate any events whatsoever, so they are not present in the macOS keytester
• Similarly, pressing the F keys doesn't generate events unless the Fn key is held or the checkbox in the keyboard preferences is ticked

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Documentation

Issues Fixed or Closed by This PR

• Fixes Add Keytester to QMK Toolbox #41

[nooges]

Tested the Mac one, seems fine so far, haven't done Windows yet.