Improve date parsing by utilizing MimeKit logic

[lahma]

Utilizing extracted code from MimeKit, removed all things not needed. No dependency on library, but attribution given in both source and CREDITS.txt.

A fast turnaround for soon three year old issue.

fixes #766

[sebastienros]

Saw this and looks good, was wondering how better it is from the logic in Fluid which I improved a lot recently for different reasons, and maybe there should be a separate library ;)

[lahma]

I tried the same test cases with Fluid using following test code, 7/8 test cases failed so I think I'll merge this now and we can revisit when yet another library emerges 😉

[Theory]
[InlineData("Thu, 30 Jan 2020 08:00:00 PST", 1580400000000)]
[InlineData("Thursday January 01 1970 00:00:25 UTC", 25000)]
[InlineData("Wednesday 31 December 1969 18:01:26 MDT", 86000)]
[InlineData("Wednesday 31 December 1969 19:00:08 EST", 8000)]
[InlineData("Wednesday 31 December 1969 17:01:59 PDT", 119000)]
[InlineData("December 31 1969 17:01:14 MST", 74000)]
[InlineData("January 01 1970 01:46:06 +0145", 66000)]
[InlineData("December 31 1969 17:00:50 PDT", 50000)]
public void CanParseLocaleString(string dateTime, long expected)
{
    var options = new TemplateOptions() { CultureInfo = new CultureInfo("en-US", useUserOverride: false), TimeZone = TimeZoneInfo.Utc };
    new StringValue(dateTime).TryGetDateTimeInput(new TemplateContext(), out var customDateTime);
    Assert.Equal(expected, FromDateTimeOffset(customDateTime));
}

internal static readonly DateTime Epoch = new(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);

private static long FromDateTimeOffset(DateTimeOffset dt, bool negative = false)
{
    var dateAsUtc = dt.ToUniversalTime();

    double result;
    if (negative)
    {
        var zero = (Epoch - DateTime.MinValue).TotalMilliseconds;
        result = zero - TimeSpan.FromTicks(dateAsUtc.Ticks).TotalMilliseconds;
        result *= -1;
    }
    else
    {
        result = (dateAsUtc - Epoch).TotalMilliseconds;
    }

    return (long) System.Math.Floor(result);
}