Use Swift 6 language mode in new target

Author: d-ronnqvist

Package.swift

@@ -122,7 +122,7 @@ let package = Package(
                 // This target shouldn't have any local dependencies so that all other targets can depend on it.
                 // We can add dependencies on SymbolKit and Markdown here but they're not needed yet.
             ],
-            swiftSettings: swiftSettings // FIXME: Use `[.swiftLanguageMode(.v6)]` here
+            swiftSettings: [.swiftLanguageMode(.v6)]
         ),
 
         .testTarget(
@@ -131,7 +131,7 @@ let package = Package(
                 .target(name: "Common"),
                 .target(name: "TestUtilities"),
             ],
-            swiftSettings: swiftSettings // FIXME: Use `[.swiftLanguageMode(.v6)]` here
+            swiftSettings: [.swiftLanguageMode(.v6)]
         ),
 
         // Test app for CommandLine

Sources/Common/SourceLanguage.swift

@@ -9,7 +9,7 @@
 */
 
 /// A programming language.
-public struct SourceLanguage: Hashable, Codable, Comparable {
+public struct SourceLanguage: Hashable, Codable, Comparable, Sendable {
     /// The display name of the programming language.
     public var name: String
     /// A globally unique identifier for the language.
@@ -132,7 +132,7 @@ public struct SourceLanguage: Hashable, Codable, Comparable {
     public static let metal = SourceLanguage(name: "Metal", id: "metal")
 
     /// The list of programming languages that are known to DocC.
-    public static var knownLanguages: [SourceLanguage] = [.swift, .objectiveC, .javaScript, .data, .metal]
+    public static let knownLanguages: [SourceLanguage] = [.swift, .objectiveC, .javaScript, .data, .metal]
 
     enum CodingKeys: CodingKey {
         case name