Use UndefOr for Node.js file Flags not supported on windows

[armanbilge]

Thanks to @sherpal for reporting! Update: they confirmed the fix works ✅

Stack trace:

<ref *2> $c_Lorg_scalajs_linker_runtime_UndefinedBehaviorError [org.scalajs.linker.runtime.UndefinedBehaviorError]: java.lang.ClassCastException: undefined cannot be cast to java.lang.Double
    at $throwClassCastException (file:///C:/Users/antoi/IdeaProjects/Ejoti/server/target/scala-3.2.0/server-fastopt/main.js:55:9)
    at $uD (file:///C:/Users/antoi/IdeaProjects/Ejoti/server/target/scala-3.2.0/server-fastopt/main.js:532:73)
    at $c_Lfs2_io_file_Flag$ (https://raw.githubusercontent.com/typelevel/fs2/v3.5.0/io/js/src/main/scala/fs2/io/file/Flag.scala:43:19)
    at $m_Lfs2_io_file_Flag$ (https://raw.githubusercontent.com/typelevel/fs2/v3.5.0/io/js/src/main/scala/fs2/io/file/Flag.scala:31:8)
    at $c_Lfs2_io_file_Flags$ (https://raw.githubusercontent.com/typelevel/fs2/v3.5.0/io/shared/src/main/scala/fs2/io/file/Flags.scala:42:20)
      at $m_Lfs2_io_file_Flag$ (https://raw.githubusercontent.com/typelevel/fs2/v3.5.0/io/js/src/main/scala/fs2/io/file/Flag.scala:31:8)
      at $c_Lfs2_io_file_Flags$ (https://raw.githubusercontent.com/typelevel/fs2/v3.5.0/io/shared/src/main/scala/fs2/io/file/Flags.scala:42:20)      at $m_Lfs2_io_file_Flags$ (https://raw.githubusercontent.com/typelevel/fs2/v3.5.0/io/shared/src/main/scala/fs2/io/file/Flags.scala:38:8)
      at $c_Lfs2_io_file_FilesCompanionPlatform$AsyncFiles.readUtf8__Lfs2_io_file_Path__Lfs2_Stream (https://raw.githubusercontent.com/typelevel/fs2/v3.5.0/io/shared/src/main/scala/fs2/io/file/Files.scala:264:71)
      at $c_Lfs2_io_file_FilesCompanionPlatform$AsyncFiles.readUtf8Lines__Lfs2_io_file_Path__Lfs2_Stream (https://raw.githubusercontent.com/typelevel/fs2/v3.5.0/io/shared/src/main/scala/fs2/io/file/Files.scala:436:15)
      at $c_Lserver_reproducer$package$.run__V (C:\Users\antoi\IdeaProjects\Ejoti\server\src\main\scala\server\reproducer.scala:13:21)
      at $s_Lserver_run__main__AT__V (C:\Users\antoi\IdeaProjects\Ejoti\server\src\main\scala\server\reproducer.scala:6:1) {
    jl_Throwable__f_s: 'undefined cannot be cast to java.lang.Double',
    jl_Throwable__f_e: null,
    jl_Throwable__f_enableSuppression: false,
    jl_Throwable__f_writableStackTrace: true,
    jl_Throwable__f_jsErrorForStackTrace: [Circular *1],
    jl_Throwable__f_stackTrace: null,
    jl_Throwable__f_suppressed: null  },
  jl_Throwable__f_enableSuppression: false,  jl_Throwable__f_writableStackTrace: true,
  jl_Throwable__f_jsErrorForStackTrace: [Circular *2],
  jl_Throwable__f_stackTrace: null,
  jl_Throwable__f_suppressed: null
}

I considered using a lazy val to defer the exception (instead of at init), but it seems that the JDK offers equivalent flags on all platforms (I assume by no-opping). So we just fallback to 0 bits set. Anyway, this saves users from having to condition use of this flag per-OS.