Simlify split2 prefix

[fgvieira]

This is not a bug, but rather a suggestion. 😄

seqkit subcommands split and split2 have several options to define prefix, depending on how the sequences are split:

      --by-length-prefix string   file prefix for --by-length. The placeholder "{read}" is needed for paired-end files.
      --by-part-prefix string     file prefix for --by-part. The placeholder "{read}" is needed for paired-end files.
      --by-size-prefix string     file prefix for --by-size. The placeholder "{read}" is needed for paired-end files.
      --by-id-prefix string       file prefix for --by-id
      --by-region-prefix string   file prefix for --by-region
      --by-size-prefix string     file prefix for --by-size

Wouldn't it be simpler to just have one prefix (e.g. --out-prefix`) for all modes?