Add ArgumentReader.Mutable#skipChar() (#2596)

Author: MrSlimeDiamond

src/main/java/org/spongepowered/api/command/parameter/ArgumentReader.java

@@ -139,6 +139,11 @@ interface Immutable extends ArgumentReader {
      */
     interface Mutable extends ArgumentReader {
 
+        /**
+         * Moves the cursor to the next character.
+         */
+        void skipChar();
+
         /**
          * Moves the cursor to the next non-whitespace character. The cursor
          * will not advance if it already refers to a non-whitespace character.