[BridgeJS] Hide it behind an experimental feature flag

Author: kateinoigakukun

Diff for: Plugins/BridgeJS/README.md

@@ -1,5 +1,8 @@
 # BridgeJS
 
+> Important: This feature is still experimental, and the API may change frequently. Use at your own risk
+> with `JAVASCRIPTKIT_EXPERIMENTAL_BRIDGEJS=1` environment variable.
+
 > Note: This documentation is intended for JavaScriptKit developers, not JavaScriptKit users.
 
 ## Overview

Diff for: Plugins/BridgeJS/Sources/BridgeJSCommandPlugin/BridgeJSCommandPlugin.swift

@@ -27,7 +27,7 @@ struct BridgeJSCommandPlugin: CommandPlugin {
                 Generated code will be placed in the target's 'Generated' directory.
 
                 OPTIONS:
-                    --target <target> Specify target(s) to generate bridge code for. If omitted, 
+                    --target <target> Specify target(s) to generate bridge code for. If omitted,
                                       generates for all targets with JavaScriptKit dependency.
                 """
         }

Diff for: Plugins/PackageToJS/Sources/PackageToJS.swift

@@ -564,6 +564,11 @@ struct PackagingPlanner {
         packageInputs.append(packageJsonTask)
 
         if exportedSkeletons.count > 0 || importedSkeletons.count > 0 {
+            if ProcessInfo.processInfo.environment["JAVASCRIPTKIT_EXPERIMENTAL_BRIDGEJS"] == nil {
+                fatalError(
+                    "BridgeJS is still an experimental feature. Set the environment variable JAVASCRIPTKIT_EXPERIMENTAL_BRIDGEJS=1 to enable."
+                )
+            }
             let bridgeJs = outputDir.appending(path: "bridge.js")
             let bridgeDts = outputDir.appending(path: "bridge.d.ts")
             packageInputs.append(