built in chords like maj maj7 #13

the built-in chords can be a zero-arity function that returns a Chord struct. for example major/0 minor/0

then in new/1 fucntion we can accept either a Chord struct or a zero-arity function, or a MFA tuple.
existing new/0 remain unchanged but should be set to private

this way user can compose their own chord shapes in however way they like

inspiration: https://github.com/wojtekmach/req/blob/dce10092b9f3b77dfa253e62a51534e8281ba0ba/lib/req/steps.ex#L122-L155