feat(builtins): add split command

[chaliy]

Context

Parity with vercel-labs/just-bash.

Requirements

• split -l N file — split file into pieces of N lines each
• split -b N file — split by byte size
• split -n N file — split into N equal pieces
• Output files named xaa, xab, etc. by default
• split -d — use numeric suffixes
• Support reading from stdin