- Run the embedder entry point directly through runEmbedderEntryPoint(),
  instead of going through another JS -> C++ trip through the
  function returned by getEmbedderEntryFunction()
- For --build-snapshot, read the snapshot script code directly in C++
  and pass it to SnapshotBuilder::Generate(), this makes the entry point
  more explicit instead of hiding it in JS land, and also makes it
  possible to invoke SnapshotBuilder::Generate() internally to create
  a custom snapshot.
- Previously we used process.execPath for the embedder to create
  __filename and __dirname in the snapshot builder script while using
  process.argv[1] for --build-snapshot (where it's always set) which
  results in inconsistencies. We now require the embedder to also set
  args[1] when creating the Environment if they intend to run snapshot
  scripts with a context that contains __filename and __dirname, which
  would be derived from args[1]. If they prefer not to include build-time
  paths in the snapshot, we now provide node::GetAnonymousMainPath()
  as an alternative.