Distinguish 6.0 and main toolchains by hasFeature(Extern)

The main toolchain still says it's 6.0, so we can' distinguish them by `#if compiler`.
The feature is baseline in the main branch but not in the 6.0, so we can use it for now.

Author: kateinoigakukun

Sources/JavaScriptEventLoop/JavaScriptEventLoop.swift

@@ -61,7 +61,7 @@ public final class JavaScriptEventLoop: SerialExecutor, @unchecked Sendable {
         return _shared
     }
 
-    #if compiler(>=6.1) && _runtime(_multithreaded)
+    #if compiler(>=6.0) && hasFeature(Extern) && _runtime(_multithreaded)
     // In multi-threaded environment, we have an event loop executor per
     // thread (per Web Worker). A job enqueued in one thread should be
     // executed in the same thread under this global executor.

Sources/JavaScriptEventLoop/WebWorkerTaskExecutor.swift

@@ -1,4 +1,4 @@
-#if compiler(>=6.1) && _runtime(_multithreaded) // @_expose and @_extern are only available in Swift 6.0+
+#if compiler(>=6.0) && hasFeature(Extern) && _runtime(_multithreaded) // @_expose and @_extern are only available in Swift 6.0+
 
 import JavaScriptKit
 import _CJavaScriptKit

Tests/JavaScriptEventLoopTests/WebWorkerTaskExecutorTests.swift

@@ -1,4 +1,4 @@
-#if compiler(>=6.1) && _runtime(_multithreaded)
+#if compiler(>=6.0) && hasFeature(Extern) && _runtime(_multithreaded)
 import XCTest
 import JavaScriptKit
 import _CJavaScriptKit // For swjs_get_worker_thread_id