Provide explicit type to Scanner.rest

Without this Scala 3 infers different type than Scala 2

Author: OndrejSpanel

shared/src/main/scala/scala/util/parsing/combinator/lexical/Scanners.scala

@@ -51,7 +51,7 @@ trait Scanners extends Parsers {
     override def source: java.lang.CharSequence = in.source
     override def offset: Int = in.offset
     def first = tok
-    def rest = new Scanner(rest2)
+    def rest: Scanner = new Scanner(rest2)
     def pos = rest1.pos
     def atEnd = in.atEnd || skip(in).atEnd
   }