feat: add `numfmt` builtin

[chaliy]

Problem

bashkit has no numfmt command. GNU numfmt converts numbers to/from human-readable format and is used in scripts for formatting disk sizes, byte counts, and other numeric output.

Expected behavior

numfmt --to=iec 1048576
# 1.0M

numfmt --from=iec 1K
# 1024

echo 1048576 | numfmt --to=si
# 1.1M

numfmt --to=iec --suffix=B 1048576
# 1.0MB

Key features to implement

• --to=si / --to=iec / --to=iec-i (output formatting)
• --from=si / --from=iec / --from=auto (input parsing)
• --suffix=SUFFIX (append suffix)
• --padding=N (pad to width)
• --round=up|down|from-zero|towards-zero|nearest (rounding mode)
• --format=FORMAT (printf-style format)
• --field=N (convert only field N from input)
• --delimiter=D (field delimiter)

Suggested approach

This is entirely pure logic — number parsing and formatting with no filesystem interaction. uutils/coreutils (MIT) has a complete implementation that could be adapted. It would live in a new numfmt.rs file in crates/bashkit/src/builtins/.

References

• GNU numfmt: https://www.gnu.org/software/coreutils/manual/html_node/numfmt-invocation.html
• uutils numfmt: https://github.com/uutils/coreutils/tree/main/src/uu/numfmt