feat(core): Add two new date/time formats and tests
feat(core): Add two new date/time formats and tests

- Added `MM/DD/YYYY HH:MM:SS [+-]HHMM`
- Added `Dow Mon DD HH:MM:SS YYYY [+-]HHMM`
- Made test for default (serialized) format check against unix time
- Added tests for using unix time for all non-default formats
- Added inline docs for `parse_date_time`

cobalt-org/liquid-rust