[number_separator] Allow year integer literals to be written without separator

Source/SwiftLintFramework/Rules/Style/NumberSeparatorRule.swift

@@ -27,9 +27,12 @@ public struct NumberSeparatorRule: OptInRule, CorrectableRule, ConfigurationProv
     private func violatingRanges(in file: File) -> [(NSRange, String)] {
         let numberTokens = file.syntaxMap.tokens.filter { SyntaxKind(rawValue: $0.type) == .number }
         return numberTokens.compactMap { (token: SyntaxToken) -> (NSRange, String)? in
-            guard let content = contentFrom(file: file, token: token),
-                isDecimal(number: content) else {
-                    return nil
+            guard
+                let content = contentFrom(file: file, token: token),
+                isDecimal(number: content),
+                !isYearLiteral(number: content)
+            else {
+                return nil
             }
 
             let signs = CharacterSet(charactersIn: "+-")
@@ -108,6 +111,14 @@ public struct NumberSeparatorRule: OptInRule, CorrectableRule, ConfigurationProv
         return prefixes.filter { lowercased.hasPrefix($0) }.isEmpty
     }
 
+    private func isYearLiteral(number: String) -> Bool {
+        if let integer = Int(number), (1500 ..< 2050).contains(integer) {
+            return true
+        }
+
+        return false
+    }
+
     private func isValid(number: String, isFraction: Bool) -> (Bool, String) {
         var correctComponents = [String]()
         let clean = number.replacingOccurrences(of: "_", with: "")