Add new keymap for GMMK with some additional mappings and led indicator for caps lock

[chofstede]

Description

New Keymap for GMMK ISO

• Added Print-Screen to FN+POS1
• Added Insert to FN+#
• The LED bars on the left and right side of the keyboard blink red when CAPS Lock is enabled
• All mapped keys light up red when FN is being pressed

Types of Changes

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

[noroadsleft]

Thanks!