refactor(keymap): Reduce flash usage for studio builds

[petejohanson]

When building for ZMK Studio, we can use the constant stock keymap for init of the in-memory keymap, to avoid duplicate flash usage for the keymap from the devicetree.

PR check-list

• Branch has a clean commit history
• Additional tests are included, if changing behaviors/core code that is testable.
• Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• Pre-commit used to check formatting of files, commit messages, etc.
• Includes any necessary documentation changes.