Implement GNU awk strftime

[dewolfe]

Summary

This adds GNU awk-compatible strftime() support. It covers the zero- through three-argument forms, the documented defaults, and PROCINFO["strftime"] overrides.

The implementation reuses the existing GNU-style date formatter. This keeps date directives consistent across date and awk.

Changes

• Format explicit Unix timestamps and use the current time when the timestamp is absent.
• Support the optional UTC flag and GNU awk's GMT zone label for forced UTC output.
• Apply awk number coercion to timestamp values and return an empty string for values outside the supported time range.
• Add focused unit tests and GNU awk-recorded comparison fixtures.
• Add the change to the unreleased changelog.

Related issue

SHL-57

Testing

• mix test — 6,818 tests and 2 doctests pass.
• mix credo — no issues.
• mix format --check-formatted — pass.
• mix bash_fixtures.verify awk — the fixture suite is sound.