Improve pretty printing of objects

If an object has more than 5 elements, it is printed in abbeviated form
displaying only the keys. This is useful in the REPL when inspecting
large objects like the Deno namespace:

  > Deno
  { args, noColor, pid, env, exit, isTTY, execPath, chdir, cwd, File,
  open, stdin, stdout, stderr, read, write, seek, close, copy,
  toAsyncIterator, SeekMode, Buffer, readAll, mkdirSync, mkdir,
  makeTempDirSync, makeTempDir, chmodSync, chmod, removeSync, remove,
  renameSync, rename, readFileSync, readFile, readDirSync, readDir,
  copyFileSync, copyFile, readlinkSync, readlink, statSync, lstatSync,
  stat, lstat, symlinkSync, symlink, writeFileSync, writeFile, ErrorKind,
  DenoError, libdeno, permissions, revokePermission, truncateSync,
  truncate, connect, dial, listen, metrics, resources, run, Process,
  inspect, build, platform, version, Console, stringifyArgs,
  DomIterableMixin }

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