Gemini parser cleanup

GeminiFormat/GeminiParser.swift

@@ -81,6 +81,13 @@ public struct GeminiParser {
     
 }
 
+fileprivate extension GeminiParser {
+    enum PreformattingState {
+        case off
+        case on(_ alt: String?)
+    }
+}
+
 fileprivate extension String {
     func firstNonWhitespaceIndex(after index: String.Index) -> String.Index {
         var index = index