support type narrowing for FileSystemHandle

[SimonSiefke]

Type narrowing for FileSystemHandles would make working with file system handles much easier

const fileHandle: FileSystemHandle = getFileHandle();

if (fileHandle.kind === "file") {
  // infer that fileHandle is a FileSystemFileHandle
} else if (fileHandle.kind === "directory") {
  // infer that fileHandle is a FileSystemDirectoryHandle
}

[saschanaz]

We need a new name for the enum since FileSystemHandle is already the name of the superclass. PR welcome if someone has a good candidate.

Any function returning FileSystemHandle should return that enum instead.