Add BlackMuller FlashBang keyboard

[JakeEMuller]

Add support for the BlackMuller FlashBang split keyboard

Description

Adds support for a new handwired keyboard, the BlackMuller Flash-Bang split keyboard, including 2 keymaps

Types of Changes

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

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).