Add unicode mode change callbacks

[spidey3]

Description

This change adds the following callbacks, to make the implementation indicators for the current unicode input mode easier:

Callback	Description
unicode_input_mode_set_kb(uint8_t input_mode)	Callback for unicode input mode set, for keyboard.
unicode_input_mode_set_user(uint8_t input_mode)	Callback for unicode input mode set, for users.

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

• none

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).