It's not often that a musical comes along that is as ambitious as it is emotional — and then succeeds on both counts. But 'Billy Elliot,' which opened Thursday at Broadway's Imperial Theatre, is an exceptional work that exemplifies what the best musicals are all about: collaboration. Everything comes together in this impressive, warmhearted adaptation of the 2000 British film about a North Country coal miner's young son who yearns to dance and join the Royal Ballet School in London.