[Experimental] Make transition configurable

[yhatt]

Updated experimental transition to make slide transitions configurable through transition local directive.

transition directive now allows YAML object. A previous way like <!-- transition: fade --> is still able to use, but YAML object is useful to specify custom duration and delay.

• type (required): The transition type.
• duration (optional): Set the duration of the transition in milliseconds between 0 to 5000.
• delay (optional): Set the delay to start the transition in milliseconds between 0 to 5000.

transition:
  type: fade
  duration: 750
  delay: 250

For better user experience, Marp CLI supports both of ms and s unit in duration and delay.

transition:
  type: fade
  duration: 0.75s
  delay: 250ms