Move `CustomRuleTimer` to its own file (#4938)

And add docs.

Source/SwiftLintFramework/Helpers/CustomRuleTimer.swift

@@ -0,0 +1,33 @@
+import Foundation
+
+/// Utility to measure the time spent in each custom rule.
+public final class CustomRuleTimer {
+    private let lock = NSLock()
+    private var ruleIDForTimes = [String: [TimeInterval]]()
+    private var shouldRecord = false
+
+    /// Singleton.
+    public static let shared = CustomRuleTimer()
+
+    /// Tell the timer it should record time spent in rules.
+    public func activate() {
+        shouldRecord = true
+    }
+
+    /// Return all time spent for each custom rule, keyed by rule ID.
+    public func dump() -> [String: TimeInterval] {
+        ruleIDForTimes.mapValues { $0.reduce(0, +) }
+    }
+
+    /// Register time spent evaluating a rule with the specified ID.
+    ///
+    /// - parameter time:   The time interval spent evaluating this rule ID.
+    /// - parameter ruleID: The ID of the rule that was evaluated.
+    func register(time: TimeInterval, forRuleID ruleID: String) {
+        guard shouldRecord else { return }
+
+        lock.lock()
+        defer { lock.unlock() }
+        ruleIDForTimes[ruleID, default: []].append(time)
+    }
+}

Source/SwiftLintFramework/Rules/Style/CustomRules.swift

@@ -1,29 +1,5 @@
 import Foundation
 
-public final class CustomRuleTimer {
-    private let lock = NSLock()
-    private var ruleIDForTimes = [String: [TimeInterval]]()
-    fileprivate var shouldRecord = false
-
-    public static let shared = CustomRuleTimer()
-
-    public func activate() {
-        shouldRecord = true
-    }
-
-    func register(time: TimeInterval, forRuleID ruleID: String) {
-        guard shouldRecord else { return }
-
-        lock.lock()
-        defer { lock.unlock() }
-        ruleIDForTimes[ruleID, default: []].append(time)
-    }
-
-    public func dump() -> [String: TimeInterval] {
-        ruleIDForTimes.mapValues { $0.reduce(0, +) }
-    }
-}
-
 private extension Region {
     func isRuleDisabled(customRuleIdentifier: String) -> Bool {
         return disabledRuleIdentifiers.contains(RuleIdentifier(customRuleIdentifier))