BridgeJS: Fix optionals build error with Embedded Swift

Remove @_spi(BridgeJS) from _BridgedAsOptional protocol and its
Optional/JSUndefinedOr conformances so the conformances are visible
in Embedded Swift mode. The underscore-prefixed protocol name already
signals internal API, and all extension methods remain @_spi-gated.

Fixes #689

Author: krodak

Sources/JavaScriptKit/BridgeJSIntrinsics.swift

@@ -182,13 +182,13 @@ extension _BridgedSwiftStackType {
 
 /// Types that bridge with the same (isSome, value) ABI as Optional.
 /// Used by JSUndefinedOr so all bridge methods delegate to Optional<Wrapped>.
-@_spi(BridgeJS) public protocol _BridgedAsOptional {
+public protocol _BridgedAsOptional {
     associatedtype Wrapped
     var asOptional: Wrapped? { get }
     init(optional: Wrapped?)
 }
 
-@_spi(BridgeJS) extension Optional: _BridgedAsOptional {
+extension Optional: _BridgedAsOptional {
     public var asOptional: Wrapped? { self }
     public init(optional: Wrapped?) { self = optional }
 }

Sources/JavaScriptKit/JSUndefinedOr.swift

@@ -53,4 +53,4 @@ extension JSUndefinedOr: ConvertibleToJSValue where Wrapped: ConvertibleToJSValu
 
 // MARK: - BridgeJS (via _BridgedAsOptional in BridgeJSIntrinsics)
 
-@_spi(BridgeJS) extension JSUndefinedOr: _BridgedAsOptional {}
+extension JSUndefinedOr: _BridgedAsOptional {}