allow mutating line and character rules

[IlyaPuchkaTW]

Right now it's not possible to easily implement the subset of markdown that would strip all unsupported markdown tags away. The simplest way to do that seem to allow mutating the styles in characters and lines rules. This way users of the library can create set of rules that will strip away unsupported tags, i.e. like this:

extension SwiftyMarkdown {
   static private let stripLineRules = SwiftyMarkdown.lineRules.map { rule -> LineRule in 
       var noStyle = rule
       noStyle.type = .body
       return noStyle
   }
   
   static private let stripCharacterRules = SwiftyMarkdown.characterRules.map { rule -> CharacterRule in
        var noStyle = rule
        noStyle.styles = rule.styles.mapValues { _ in
            CharacterStyle.none
        }
        return noStyle
    }
}