[Core] Add a new finding severity - refactoring

Refactoring identifies an actionable feedback to the developer and
has higher impact than a general warning.

Author: xedin

Sources/SwiftFormatCore/Finding.swift

@@ -16,6 +16,7 @@ public struct Finding {
   public enum Severity {
     case warning
     case error
+    case refactoring
   }
 
   /// The file path and location in that file where a finding was encountered.

Sources/SwiftFormatCore/Rule.swift

@@ -46,6 +46,7 @@ extension Rule {
   public func diagnose<SyntaxType: SyntaxProtocol>(
     _ message: Finding.Message,
     on node: SyntaxType?,
+    severity: Finding.Severity? = nil,
     leadingTriviaIndex: Trivia.Index? = nil,
     notes: [Finding.Note] = []
   ) {
@@ -57,9 +58,10 @@ extension Rule {
       syntaxLocation = node?.startLocation(converter: context.sourceLocationConverter)
     }
 
+    let category = RuleBasedFindingCategory(ruleType: type(of: self), severity: severity)
     context.findingEmitter.emit(
         message,
-        category: RuleBasedFindingCategory(ruleType: type(of: self)),
+        category: category,
         location: syntaxLocation.flatMap(Finding.Location.init),
         notes: notes)
   }

Sources/SwiftFormatCore/RuleBasedFindingCategory.swift

@@ -22,8 +22,11 @@ struct RuleBasedFindingCategory: FindingCategorizing {
 
   var description: String { ruleType.ruleName }
 
+  var severity: Finding.Severity?
+
   /// Creates a finding category that wraps the given rule type.
-  init(ruleType: Rule.Type) {
+  init(ruleType: Rule.Type, severity: Finding.Severity? = nil) {
     self.ruleType = ruleType
+    self.severity = severity
   }
 }

Sources/swift-format/Utilities/DiagnosticsEngine.swift

@@ -116,6 +116,7 @@ final class DiagnosticsEngine {
     switch finding.severity {
     case .error: severity = .error
     case .warning: severity = .warning
+    case .refactoring: severity = .warning
     }
     return Diagnostic(
       severity: severity,