Keep track of encoder activity

[tzarc]

Description

Addition to the "last matrix activity" PR, this time around it keeps track of encoder activity too.

Adds four new APIs:

uint32_t last_input_activity_time(void);     // Timestamp of the last matrix or encoder activity
uint32_t last_input_activity_elapsed(void);  // Number of milliseconds since the last matrix or encoder activity

uint32_t last_encoder_activity_time(void);     // Timestamp of the last encoder activity
uint32_t last_encoder_activity_elapsed(void);  // Number of milliseconds since the last encoder activity

Tested on the Djinn rev1.

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).

[tzarc]

Interesting bug:

https://github.com/tzarc/qmk_firmware/blob/1274f566458fb64351202447fca145853730fa53/tmk_core/common/keyboard.c#L395...L412

Ends up with the following build error when there's no RGBLIGHT or Backlight enabled:

Compiling: tmk_core/common/keyboard.c                                                              In file included from tmk_core/common/keyboard.h:20,
                 from tmk_core/common/keyboard.c:19:
tmk_core/common/keyboard.c: In function 'keyboard_task':
tmk_core/common/keyboard.c:412:5: error: a label can only be part of a statement and a declaration is not a statement
  412 |     bool encoders_changed = encoder_read();
      |     ^~~~
 [ERRORS]