Add partial support for datetime() function

[PThorpe92]

This PR adds the datetime function, with all the support currently that date/time have for modifiers, and julianday function, as well as some additional modifiers for date/time/datetime.

There are a couple considerations here, I left a couple comments but essentially there is going to have to be some more work done to track the state of the expression during the application of modifiers, to handle a bunch of edge-cases like re-applying the same timezone modifier to itself, or converting an integer automatically assumed to be julianday, into epoch, or ceiling/floor which will determine relative addition of time in cases like

2024-01-31 +1 month = 2024-03-02

which was painful enough to get working to begin with.

I couldn't get the julianday_converter library to get the exact same float precision as sqlite, so function is included that matches their output, for some reason floating point math + .floor() would give the correct result. They seem to 'round' to 8 decimal places, and I was able to get this to work with the same output as sqlite, except in cases like 2234.5, in which case we return 2234.5000000 because of the fmt precision:

pub fn exec_julianday(time_value: &OwnedValue) -> Result<String> {
    let dt = parse_naive_date_time(time_value);
    match dt {
        // if we did something heinous like: parse::<f64>().unwrap().to_string()
        // that would solve the precision issue, but dear lord...
        Some(dt) => Ok(format!("{:.1$}", to_julian_day_exact(&dt), 8)),
        None => Ok(String::new()),
    }
}

Suggestions would be appreciated on the float precision issue.

[sonhmai]

thanks!

[sonhmai]

great! adding the time modifiers makes it very clear 🥇

[sonhmai]

added #602 for supporting modifiers for these functions later.