[Keyboard] Helix add split common option (#7718)

* is_master, has_usb() move to rev2.[hc]

* Do recent helix/rev2 changes to helix/pico as well.

helix/pico/matrix.c: remove 'is_master'
helix/pico/pico.c: add 'is_master'
helix/pico/pico.h: add 'has_usb()' macro
helix/pico/split_util.c: remove 'setup_handedness()' 'has_usb()', add 'is_helix_master()' etc

* add HELIX=scan option into {rev2/pico}/local_features.mk

Made DEBUG_MATRIX_SCAN_RATE easy to use.

* Changed rules.mk to link "helix/local_drivers/ssd1306.c" only when OLED_ENABLE = yes.

* Added option to use split_common for helix/rev2, helix/pico keyboard.

how to build:
 ### build helix/pico (HelixPico) with helix current codes
  $ make helix/pico:KEY_MAP
  $ make helix/pico/back:KEY_MAP

 ### build helix/rev2 (Helix or Helix beta) with helix current codes
  $ make helix:KEY_MAP
  $ make helix/rev2/back:KEY_MAP
  $ make helix/rev2/under:KEY_MAP
  $ make helix/rev2/oled:KEY_MAP
  $ make helix/rev2/oled/back:KEY_MAP
  $ make helix/rev2/oled/under:KEY_MAP

 ### build helix/pico (HelixPico) with split_common codes
  $ make helix/pico/sc:KEY_MAP
  $ make helix/pico/sc/back:KEY_MAP
  $ make helix/pico/sc/under:KEY_MAP

 ### build helix/rev2 (Helix) with split_common codes
  $ make helix/rev2/sc:KEY_MAP
  $ make helix/rev2/sc/back:KEY_MAP
  $ make helix/rev2/sc/under:KEY_MAP
  $ make helix/rev2/sc/oled:KEY_MAP
  $ make helix/rev2/sc/oledback:KEY_MAP
  $ make helix/rev2/sc/oledunder:KEY_MAP

* add matrix_slave_scan_user() to helix/rev2/rev2.c, helix/pico/pico.h

* Changed 'helix:xulkal' to always use split_common and removed ad hoc code.

Added the following line to 'helix/rev2/keymaps/xulkal/rules.mk':

        SPLIT_KEYBOARD = yes

Removed the following ad hoc code from 'users/xulkal/custom_oled.c':

        #if KEYBOARD_helix_rev2
        extern uint8_t is_master;
        bool is_keyboard_master(void) { return is_master; }
        #endif

* add '#define DIODE_DIRECTION COL2ROW' into helix/{rev2|pico}/config.h

This commit does not change the build result.

* update helix readme

* keyboards/helix/readme.md
* keyboards/helix/pico/keymaps/default/readme.md
* keyboards/helix/rev2/keymaps/default/readme.md

Co-authored-by: mtei <2170248+mtei@users.noreply.github.com>

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