Documentation: implement edition using YearMonth

Also, add the toString method so that the automatic documentation which
lists all possible options displays this value in a more user-friendly
way.

scalafmt-core/shared/src/main/scala/org/scalafmt/config/Edition.scala

@@ -1,24 +1,33 @@
 package org.scalafmt.config
 
+import java.time.YearMonth
+
 import metaconfig.{Conf, ConfDecoder, ConfEncoder, Configured}
 
-case class Edition(year: Int, month: Int)
+class Edition(val ym: YearMonth) extends AnyVal {
+  override def toString: String = ym.toString
+}
 
 object Edition {
-  val Latest = Edition(Int.MaxValue, Int.MaxValue)
+
+  def apply(year: Int, month: Int): Edition =
+    new Edition(YearMonth.of(year, month))
+
+  val Latest = Edition(9999, 12)
+
   implicit val ordering: Ordering[Edition] =
-    Ordering.by[Edition, (Int, Int)](e => e.year -> e.month)
+    (x: Edition, y: Edition) => x.ym.compareTo(y.ym)
+
   lazy val format = "(\\d{4})-(\\d{1,2})".r
 
   implicit val decoder: ConfDecoder[Edition] =
     ConfDecoder.instanceExpect("'$year-$month', for example '2019-08'") {
       case Conf.Str(format(year, month)) =>
-        Configured.ok(Edition(year.toInt, month.toInt))
+        try Configured.ok(Edition(year.toInt, month.toInt))
+        catch { case e: Exception => Configured.error(e.getMessage) }
       case Conf.Str("latest") => Configured.ok(Latest)
     }
 
   implicit val encoder: ConfEncoder[Edition] =
-    ConfEncoder.instance(edition =>
-      Conf.Str(f"${edition.year}%d-${edition.month}%02d")
-    )
+    ConfEncoder.instance(edition => Conf.Str(edition.ym.toString))
 }

scalafmt-tests/src/test/scala/org/scalafmt/EditionTest.scala

@@ -6,14 +6,15 @@ import org.scalafmt.config.Edition
 import org.scalafmt.util.DiffAssertions
 
 class EditionTest extends AnyFunSuite with DiffAssertions {
-  def check(original: String, expected: Edition): Unit = {
+  def check(original: String, expected: Edition, expectedStr: String): Unit = {
     test(original) {
       val conf = Conf.Str(original)
       val obtained = Edition.decoder.read(conf).get
       assertNoDiff(
         Edition.encoder.write(obtained).toString(),
         Edition.encoder.write(expected).toString()
       )
+      assertNoDiff(expected.toString, expectedStr)
     }
   }
   def checkError(original: String, expectedError: String): Unit = {
@@ -26,9 +27,9 @@ class EditionTest extends AnyFunSuite with DiffAssertions {
       }
     }
   }
-  check("2019-09", Edition(2019, 9))
-  check("2019-9", Edition(2019, 9))
-  check("2019-10", Edition(2019, 10))
+  check("2019-09", Edition(2019, 9), "2019-09")
+  check("2019-9", Edition(2019, 9), "2019-09")
+  check("2019-10", Edition(2019, 10), "2019-10")
   checkError(
     "2019-invalid",
     """|Type mismatch;