https://dictionary.cambridge.org/us/dictionary/english/too too adverb (CERTAINLY) US informal used to emphasize a positive answer to a negative statement: "I'm not going to school today." "You are too!" ("You are too going to school!") The bet is to NOT jump into the pool. "My God, you would too jump into the pool!" The statement expresses surprise that person B would jump into the pool even though the bet was to not jump in.

no it doesn't mean that it is an expression you are basically saying that actions is something only you would do for example you may have a crazy friend that likes to do really crazy things. after he eats like a bowl of hot peppers if he tells you "hey man I just ate all those peppers" you might laughingly say "haha you would too"

It's more of an expression of surprise like 'Oh my gosh you really would.' It's rarely seen in a written form, more speech between friends.