Add combiners field for layouts

java/src/org/futo/inputmethod/event/CombinerChain.java

java/src/org/futo/inputmethod/keyboard/KeyboardSwitcher.java

java/src/org/futo/inputmethod/latin/LatinIMELegacy.java

java/src/org/futo/inputmethod/latin/WordComposer.java

java/src/org/futo/inputmethod/v2keyboard/CombinerKind.kt

java/src/org/futo/inputmethod/v2keyboard/Keyboard.kt