edit: group in-sequence, single-rune edits by the same editor into the same undo frame

[eaburns]

When a user is typing, they enter one rune at a time. Typically, we would expect an Undo to undo the entire typing event, not just the last rune typed. Edit should detect in-sequences, single-rune edits made by the same editor and put them in the same undo frame so they are undone together.