Undo stacks are not granular enough for Korean

Testing #229383

Right now, typing a sentence in Korean and pressing undo will remove the entire line. I confirmed with a native Korean speaker that the ideal behavior for undo is to remove the last consonant/vowel (each character is made up of 2-4 consonants/vowels). This might be difficult to do depending on how EditContext words, but a little less better but much better than deleting the whole line is to delete the last fully formed character.

Note that this is not a regression, but it's a pretty bad existing great experience.