[Keymap] Add new IIDX gamepad keymap for Synth Labs Solo

[hongaaronc]

Description

These changes add another keymap, which makes this macropad function as a gamepad which works out-of-the-box with the game beatmania IIDX INFINITAS.

I have tested this keymap manually in-game, and verified that the behavior works as expected. Please also refer to the new README to see what the expected behaviors are.

The rotary encoder in this keymap is intended to have slightly different behavior from the most standard rotary encoder implementation:

• In the most rotary encoder implementations, spinning an encoder continuously generally outputs repeated taps of a keycode
• In this implementation, spinning an encoder continuously outputs a keycode being pressed and held until the spinning stops
• Additionally, if a spinning encoder's direction changes, release the keycode corresponding to the previous direction immediately

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

• N/A

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[hongaaronc]

Thanks for looking over the changes and for the approvals. Looks like I don’t have authorization to merge though, so could someone help merge them for me? I don’t have anything further that I intend to change on this PR, so should be good to go from my side.

[hongaaronc]

Thanks!