Fix NullPointerException when using fileName under Scala 3 repl (#153)

#125

Wasn't able to run tests locally - kept seeing failures with mill
script.

sourcecode/src-3/sourcecode/Macros.scala

@@ -152,7 +152,7 @@ object Macros {
   }
 
   def fileNameImpl(using Quotes): Expr[sourcecode.FileName] = {
-    val name = quotes.reflect.Position.ofMacroExpansion.sourceFile.jpath.getFileName.toString
+    val name = quotes.reflect.Position.ofMacroExpansion.sourceFile.name
     '{sourcecode.FileName(${Expr(name)})}
   }