Add a mini shell

[ryuukumar]

Adds cosh, a tiny shell that is capable of:

• running builtin commands chdir, cd, echo, eval, exit, getpid, ls, mkdir, pwd, source, stat, touch
• running elf binaries in the current directory with the typical fork + exec + wait structure
• (untested) running elf binaries that are accessible by parsing PATH
• elementary shell parsing like comments, semicolons, replacing $$ and $?
• elementary conditional chaining with && and ||
• builtin test with [ alias, with support for:
  • single arg, return true if exists
  • two arg (! does not contribute to arg count), check modes z, n, e, f, d, s
  • three arg, string comparisons = and !=, integer comparisons -eq, -ne, -lt, -le, -gt, -ge
• interpreting passed file (syntax cosh <file>)

[ryuukumar]

Basic shell works!