feat(builtins): add join command

[chaliy]

Context

Parity with vercel-labs/just-bash.

Requirements

• join file1 file2 — join lines of two sorted files on a common field
• join -1 N -2 M — join on field N of file1 and field M of file2
• join -t CHAR — field separator
• join -a 1 / join -a 2 — include unpairable lines
• join -e STR — replace missing fields with STR
• join -o FORMAT — output format specification
• Input files must be sorted on the join field