Update scalafmt configuration for Scala 3 (#679)

- Change dialect from scala213source3 to scala3
- Add StandardConvention project layout
- Reduce maxColumn from 180 to 100 for better readability
- Add Scala 3 specific rewrite rules for modern syntax
- Configure newlines and docstring formatting

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <[email protected]>

Author: xerial

.scalafmt.conf

@@ -1,5 +1,16 @@
 version = 3.9.4
-runner.dialect = scala213source3
-maxColumn = 180
+project.layout = StandardConvention
+runner.dialect = scala3
+maxColumn = 100
 style = defaultWithAlign
-optIn.breaksInsideChains = true
+docstrings.blankFirstLine = yes
+rewrite.scala3.convertToNewSyntax = true
+rewrite.scala3.removeOptionalBraces = yes
+rewrite.scala3.insertEndMarkerMinLines = 30
+# Add a new line before case class
+newlines.topLevelStatementBlankLines = [
+  {
+    blanks { after = 1 }
+  }
+]
+newlines.source = unfold