fix: percent in enum values

[corani]

Translate % in enum values to Percent. E.g. the following model:

funny:
  type: string
  enum: [ '%' ]

results in:

const (
    FunnyPercent Funny = "%"
)

Closes: #661

[corani]

This should fix #661, however it feels a bit like whack-a-mole. There's a long list of other characters that are dropped.