Scala parser: map def declarations to J.MethodDeclaration

[jkschneider]

Summary

• Maps Scala def method declarations to J.MethodDeclaration instead of J.Unknown
• Adds ScalaPrinter.visitMethodDeclaration to print Scala syntax (def name(params): ReturnType = body)
• Adds visitMethodParameter to parse method parameters as J.VariableDeclarations
• Adds visitDefDefImpl with proper handling of modifiers, type parameters, return types, and bodies
• Single-expression bodies use OmitBraces marker
• Falls back to J.Unknown for edge cases: annotated methods, procedure syntax, complex parameter types (Map[String, Any], function types), parameterless methods, nested braces, while loops in body

Test plan

• All 8 MethodDeclarationTest cases pass (simple, params, default params, type params, abstract, private, override, multi-param)
• All existing tests pass with zero regressions (33 previously affected tests fixed)