add new mruby-strftime gem providing time#strftime for formatting
time objects using standard format specifiers.
implementation features:
- uses mrb_time_get_tm() api for accessing time components
- handles nul bytes in format strings correctly
- dynamic buffer allocation for variable-length output
- comprehensive test coverage including edge cases
Co-authored-by: Claude <noreply@anthropic.com>
Yukihiro "Matz" Matsumoto