Output a warning for unknown config rules

**Summary**:
- Helps developers surface any errors and typos in their `.swift-format`
  config files by showing a warning when running `swift-format` if the
  configuration includes invalid rules.
- The actual check is in `Frontend.swift`.
- The dictionary of rules for verification is in `RuleRegistry`, which
  is made `public` to be able to import it in `swift-format` target.

**Motivation**:
- Closes swiftlang#607

Sources/SwiftFormatConfiguration/RuleRegistry+Generated.swift

@@ -12,8 +12,8 @@
 
 // This file is automatically generated with generate-pipeline. Do Not Edit!
 
-enum RuleRegistry {
-  static let rules: [String: Bool] = [
+public enum RuleRegistry {
+  public static let rules: [String: Bool] = [
     "AllPublicDeclarationsHaveDocumentation": false,
     "AlwaysUseLowerCamelCase": true,
     "AmbiguousTrailingClosureOverload": true,

Sources/generate-pipeline/RuleRegistryGenerator.swift

@@ -40,8 +40,8 @@ final class RuleRegistryGenerator: FileGenerator {
 
       // This file is automatically generated with generate-pipeline. Do Not Edit!
 
-      enum RuleRegistry {
-        static let rules: [String: Bool] = [
+      publc enum RuleRegistry {
+        public static let rules: [String: Bool] = [
 
       """
     )

Sources/swift-format/Frontend/Frontend.swift

@@ -181,7 +181,9 @@ class Frontend {
     // loaded. (Do not try to fall back to a path inferred from the source file path.)
     if let configurationFileURL = configurationFileURL {
       do {
-        return try configurationLoader.configuration(at: configurationFileURL)
+        let configuration = try configurationLoader.configuration(at: configurationFileURL)
+        self.verifyConfigurationRules(for: configuration)
+        return configuration
       } catch {
         diagnosticsEngine.emitError("Unable to read configuration: \(error.localizedDescription)")
         return nil
@@ -193,6 +195,7 @@ class Frontend {
     if let swiftFileURL = swiftFileURL {
       do {
         if let configuration = try configurationLoader.configuration(forSwiftFileAt: swiftFileURL) {
+          self.verifyConfigurationRules(for: configuration)
           return configuration
         }
         // Fall through to the default return at the end of the function.
@@ -208,4 +211,16 @@ class Frontend {
     // default configuration.
     return Configuration()
   }
+
+  /// Checks if all the rules in the given configuration are supported by the registry.
+  /// If there are any rules that are not supported, they are emitted as a warning.
+  private func verifyConfigurationRules(for configuration: Configuration) -> Void {
+    // If any rules in the decoded configuration are not supported by the registry,
+    // emit them into the diagnosticsEngine as warnings.
+    // That way they will be printed out, but we'll continue execution on the valid rules.
+    let invalidRules = configuration.rules.filter { !RuleRegistry.rules.keys.contains($0.key) }
+    if !invalidRules.isEmpty {
+      diagnosticsEngine.emitWarning("Configuration contains an unsupported rule: \(invalidRules.keys.joined(separator: ", "))", location: nil)
+    }
+  }
 }

Sources/swift-format/Utilities/DiagnosticsEngine.swift

@@ -65,6 +65,20 @@ final class DiagnosticsEngine {
         message: message))
   }
 
+  /// Emits a generic warning message.
+  ///
+  /// - Parameters:
+  ///   - message: The message associated with the error.
+  ///   - location: The location in the source code associated with the error, or nil if there is no
+  ///     location associated with the error.
+  func emitWarning(_ message: String, location: SourceLocation? = nil) {
+    emit(
+      Diagnostic(
+        severity: .warning,
+        location: location.map(Diagnostic.Location.init),
+        message: message))
+  }
+
   /// Emits a finding from the linter and any of its associated notes as diagnostics.
   ///
   /// - Parameter finding: The finding that should be emitted.