[Feature] add long press callback interval

[wdllmh]

Summary

1. Adds macro LONG_CALLBACK_TICKS, provide an interval between two callback function BTN_LONG_PRESS_HOLD called.
2. Updates test case 12 to cover and verify this new feature.
3. Fixes an inaccuracy state machine diagram in both README.md.

Changes

Feature

• Long press state contains idle -> press -> long-press-hold -> idle, so reset of handle->tick won't interfere with other state.
• Uses conditional compilation macro to provide optional use

Tests

• make sure state machine has entered BTN_STATE_LONG_HOLD before manually setting ticks near UINT16_MAX

Testing

• make test passes
• make all compiles without warnings
• Tested on hardware (if applicable): implemented on STM32F103C8T6, using long press to change lamp brightness continuously. Use this interval to match button tick and lamp tick.
• Flash usage increase by 40 Byte after setting LONG_CALLBACK_TICKS larger than 1

P.S. This is my first PR. I'm very excited to contribute! Any feedback or suggestions for improvement are greatly appreciated.