Timer Face: Advanced countdown face with presets (#224)

* timer face: initial commit, fully functional

* timer face: show slot number in normal mode

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Co-authored-by: joeycastillo <joeycastillo@utexas.edu>

movement/movement_faces.h

@@ -77,6 +77,7 @@
 #include "discgolf_face.h"
 #include "habit_face.h"
 #include "repetition_minute_face.h"
+#include "timer_face.h"
 // New includes go above this line.
 
 #endif // MOVEMENT_FACES_H_