Make chrono optional

Chrono is a relatively big dependency, at least making it optional would allow user to choose to remove it

cargo bloat: chrono alone is adding 1.1% to text section

File  .text    Size    Crate Name
0.1%   1.2% 19.4KiB      std addr2line::ResDwarf<R>::parse
0.1%   1.1% 18.4KiB      std std::backtrace_rs::symbolize::gimli::resolve::{{closure}}
0.1%   0.7% 11.7KiB reedline reedline::engine::Reedline::handle_editor_event
0.1%   0.6% 10.7KiB reedline reedline::main
0.0%   0.6%  9.5KiB      std addr2line::ResUnit<R>::parse_lines
0.0%   0.6%  9.4KiB   chrono <chrono::format::strftime::StrftimeItems as core::iter::traits::iterator::Iterator>::next
0.0%   0.5%  8.5KiB      std miniz_oxide::inflate::core::decompress
0.0%   0.5%  7.6KiB   chrono chrono::format::format_inner

here is an example implementation https://github.com/nushell/reedline/compare/main...sigmaSd:opt_chrono?expand=1

comparison with this pr

# with chrono
cargo tree | wc -l
> 88

# without chrono
 cargo tree --no-default-features | wc -l
> 77

Review the linked opt_chrono comparison first, then inspect the repository's current Cargo feature and dependency configuration. Compare cargo tree output with and without the optional dependency, and verify that the project still builds and works when chrono is disabled.