Send Fn scancode to OS on press and release

[smumriak]

Proposed change implements return of EC_SUCCESS from keyboard_scancode_callback on press and release of Fn key. This is needed in order to send the the scancode to OS. This is primarily quality of life change since it would allow the OS to bind Fn key to some other modifier. Similar functionality exists on macOS for Apple Macbooks. From personal experience I've found remapping of Fn to Ctrl extremely useful, as it allows my muscle memory to work similarly on both Framework and Macbook laptops. I am not convinced that his commit is enough tho, since the Fn hotkeys are technically sending completely different scancoded for actions (think of Home/End/PgDown/PgUp). OS might recognize those as used while the bound modifier is pressed. The obvious solution would be to send three scancodes in a row: release Fn, hotkey scancode, press Fn. I would expect EC firmware implement this on its own, but have not verified that yet. Feedback is very welcome

[github-actions[bot]]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[smumriak]

I have read the CLA Document and I hereby sign the CLA

[smumriak]

Clising this PR for two reasons:

• linux is not able to understand scancode of 0x00ff, at least not for now
• CrOS EC does not pass 0x00ff scancode to OS because it can not translate it. From EC's point of view it's unknown scancode that does not belong to any known scancode set

Much more changes needed to enable aforementioned behavior.