Improve shell script functionality

[nbeerbower]

New functions:

• sh(cmd): Run shell string directly, return stdout (throws on failure)
• sh_ok(cmd): Run shell string, return bool success status
• last_exit(): Get exit code from last sh/sh_ok call
• range(start, end, step?): Generate numeric ranges like {1..10}
• char_range(start, end): Generate character ranges like {a..z}
• expand(pattern): Bash-style brace expansion

Features:

• List expansion: {a,b,c} -> ["a", "b", "c"]
• Numeric ranges: {1..5} -> ["1", "2", "3", "4", "5"]
• Character ranges: {a..f} -> ["a", "b", "c", "d", "e", "f"]
• With prefix/suffix: file{1..3}.txt -> ["file1.txt", ...]
• Cartesian product: {a,b}{1,2} -> ["a1", "a2", "b1", "b2"]
• Zero-padding: {01..05} -> ["01", "02", "03", "04", "05"]
• Step ranges: {1..10..2} -> ["1", "3", "5", "7", "9"]

Includes parity tests and updated documentation.