emoji_eventsLine diff (longest common subsequence)

Write diffLines(a, b) that compares two arrays of lines and returns an edit script: an array of [op, line] pairs where op is "=" (in both), "-" (only in a) or "+" (only in b).

• Keeping the "=" lines must give the longest common subsequence, so the diff is as small as possible.
• Taking "=" and "-" lines in order rebuilds a; taking "=" and "+" lines rebuilds b.
• Arrays up to a few hundred lines must finish quickly (dynamic programming, O(n·m)).

Acceptance criteria

• The tests at the bottom print All tests passed.