Expose swjs_library_features from Swift side

We now support only Swift 6.0 and above, so we can use `@_expose(wasm)`
instead of `__attribute__((export_name))` and `@_cdecl` hack.

Author: kateinoigakukun

Diff for: Sources/JavaScriptKit/Features.swift

@@ -2,17 +2,13 @@ enum LibraryFeatures {
     static let weakRefs: Int32 = 1 << 0
 }
 
-@_cdecl("_library_features")
-func _library_features() -> Int32 {
+@_expose(wasm, "swjs_library_features")
+@_cdecl("_swjs_library_features")
+@available(*, unavailable)
+public func _swjs_library_features() -> Int32 {
     var features: Int32 = 0
 #if !JAVASCRIPTKIT_WITHOUT_WEAKREFS
     features |= LibraryFeatures.weakRefs
 #endif
     return features
 }
-
-#if compiler(>=6.0) && hasFeature(Embedded)
-// cdecls currently don't work in embedded, and expose for wasm only works >=6.0
-@_expose(wasm, "swjs_library_features")
-public func _swjs_library_features() -> Int32 { _library_features() }
-#endif

Diff for: Sources/_CJavaScriptKit/_CJavaScriptKit.c

@@ -46,13 +46,6 @@ void swjs_free_host_function(const JavaScriptHostFuncRef host_func_ref) {
     _free_host_function_impl(host_func_ref);
 }
 
-int _library_features(void);
-
-__attribute__((export_name("swjs_library_features")))
-int swjs_library_features(void) {
-    return _library_features();
-}
-
 int swjs_get_worker_thread_id_cached(void) {
     _Thread_local static int tid = 0;
     if (tid == 0) {