Generating ISO8601 or RFC3339 string from Time

[JanBeelte]

Hello,

to me a convenience method to get write ISO8601 or RFC3339 string from Time seems like a good idea.
Would you accept a PR for that?
If not a sample in the Readme would also be convenient :)

[dennypenta]

you probably look for something like this, not sure it exists in the readme, found in the source:

        const zIso = try zeit.instant(.{
            .source = .{
                .rfc3339 = "2023-01-15T23:45:51Z",
            },
        });
        const ns = zIso.timestamp;
        const zNs = try zeit.instant(.{
            .source = .{
                .unix_nano = ns,
            },
        });
        var timeBuf: [24]u8 = undefined;
        const time = try zNs.time().bufPrint(&timeBuf, .rfc3339);
        try std.testing.expectEqualStrings(c.want, time);

[rockorager]

@dennypenta is right for RFC3339. The hard part for ISO8601 is that there are lots of formats defined there - but I would accept a PR that adds common variants to the Format enum and implemented in bufPrint