stream: add file_mode option

[isbee]

Related issue: #1056

Add a --file_mode option to the stream example. This feature works together with the --file option.

• raw: Writes every iteration, so it captures pre‑newline interim tokens, which are continuously overwritten on the terminal. (noisy, current behavior)
• newline: Buffers and writes only at the newline boundary, This drops all pre-newline tokens and records only the finalized line.
  • With a high n_new_line value (large length/step), this mode behaves quite differently from raw.
  • With a low n_new_line value, the difference is smaller.

Origin issue mentioned clearing last line of file. This might be implemented with overwrite mode in the future.