Add modifier state to the split keyboard transport (#10400)

* Add modifier state to the split transport

This adds modifier state to the i2c and serial transport for split
keyboards.  The purpose of this is to allow e.g. displaying modifier
state on the slave side of a split keyboard on an oled.  This adds one
byte to the data transferred between halves.

This also fixes a missing ifdef guard for BLACKLIGHT_ENABLE.

Break modifiers into real/weak/oneshot

Fix incorrect slave serial mod setting

Fix typo in serial weal mod setter

Fix build errors for the I2C code that I introduced

Code cleanup and formatting per project preferences

Correctly get oneshot mods

Fix missing braces

Remove unneeded ifdef guard

Make the added state transport optional

Add documentation for the new define to enable this feature

Fix stray grave mark

* Fix error introduced in conflict resolution

1 files changed, 10 insertions, 0 deletions

diff --git a/docs/feature_split_keyboard.md b/docs/feature_split_keyboard.md
index b234114200..c285e353d4 100644
--- a/docs/feature_split_keyboard.md
+++ b/docs/feature_split_keyboard.md
@@ -181,6 +181,16 @@ If you're having issues with serial communication, you can change this value, as
 * **`4`**: about 26kbps
 * **`5`**: about 20kbps
 
+```c
+#define SPLIT_MODS_ENABLE
+```
+
+This enables transmitting modifier state (normal, weak and oneshot) to the non
+primary side of the split keyboard.  This adds a few bytes of data to the split
+communication protocol and may impact the matrix scan speed when enabled.
+The purpose of this feature is to support cosmetic use of modifer state (e.g.
+displaying status on an OLED screen).
+
 ###  Hardware Configuration Options
 
 There are some settings that you may need to configure, based on how the hardware is set up.