Add enums for sc2 macro definitions

[JustNoon]

Adding global enums to macro definitions would let code, documentation, and argument validation reuse values for more robust and convenient documentation.

sugarcube-2:
  enums:
    colors: '"red"|"green"|"blue"|"pink"'
  macros:
    coloredbutton:
      name: coloredbutton
      description: |-
        `<<coloredbutton label color>>`

        `label`: What the button says

        `color`: %colors%
      parameters:
        - 'text &+ %colors%'

Global enums can be evaluated when requested or cached after all files evaluated.

Local would be useful too if global are too much. Thoughts?