Bring pretty printer up to date

The pretty printer converts a Regex defined in syntax into a RegexBuilder-style one, but it isn't up to date with the latest changes. This needs to be brought up to date, ideally with a validator that can take a regex and some input/output data, convert the regex to builder style, and then validate that both regexes have the same behavior.

For example, #"(?P)\w+ \d{2,} -"# converts to

Group(/* TODO: changeMatchingOptions */) {
  Concatenation {
    /* TODO: assertions */
    OneOrMore(.eager) {
      .wordCharacter
    }
    "-"
  }
}

instead of

Regex {
  Anchor.startOfInput
  OneOrMore(.eager) {
    CharacterClass.wordCharacter
  }
  "-"
}.asciiOnlyCharacterClasses()