x.json2: add encoder support for time.Time

[Delta456]

This PR adds support for time.Time for x.json2,'s encode_any function.

Code:

import x.json2
import time

fn main() {
    println(json2.encode({
        'bro': json2.Any(time.now())
    }))
}

Output:

{"bro":"2023-12-20T12:32:50.379Z"}