Allow editing *.mill files

This is an follow-up to scala-steward-org#3410 (comment)

Author: fthomas

docs/repo-specific-configuration.md

@@ -130,7 +130,7 @@ updates.allowPreReleases  = [ { groupId = "com.example", artifactId="foo" } ]
 updates.limit = 5
 
 # The extensions of files that should be updated.
-# Default: [".mill-version",".sbt",".sbt.shared",".sc",".scala",".scalafmt.conf",".sdkmanrc",".yml","build.properties","mill-version","pom.xml"]
+# Default: [".mill",".mill-version",".sbt",".sbt.shared",".sc",".scala",".scalafmt.conf",".sdkmanrc",".yml","build.properties","mill-version","pom.xml"]
 updates.fileExtensions = [".scala", ".sbt", ".sbt.shared", ".sc", ".yml", ".md", ".markdown", ".txt"]
 
 # If "on-conflicts", Scala Steward will update the PR it created to resolve conflicts as

modules/core/src/main/scala/org/scalasteward/core/repoconfig/UpdatesConfig.scala

@@ -92,6 +92,7 @@ final case class UpdatesConfig(
 object UpdatesConfig {
   val defaultFileExtensions: Set[String] =
     Set(
+      ".mill",
       MillAlg.millVersionName,
       MillAlg.millVersionNameInConfig,
       ".sbt",