Bugfix chrono panic + hotifx PR15544 (#15549)

Closes  #13972

# Description
First commit: a hotfix concerning my last PR #15544! I had a
``unwrap_or_default`` that resulted in all years before ~1800 being
considered as "now", because the ``num_nanoseconds()`` overflowed.
Cc @fdncred 

Second: about #13972
Negative years are not allowed with RFC 2822 formatting, so I fallback
RTC 3339 in such cases.

If you want you might Rebase and Merge, and not squash.

# User-Facing Changes
On master 🔴 :
```nu
~> {year: 1900} | into datetime
Mon, 1 Jan 1900 00:00:00 +0200 (125 years ago)
# OK

~> {year: 1000} | into datetime
Wed, 1 Jan 1000 00:00:00 +0200 (now)
# NOT OK: now?

~> {year: -1000} | into datetime
-1000-01-01T00:00:00+02:00 (now)
# NOT OK: now?

~> {year: -1000} | into datetime | format date 
Error:   × Main thread panicked.
  ├─▶ at C:\Users\RIL1RT\.cargo\registry\src\index.crates.io-6f17d22bba15001f\chrono-0.4.39\src\datetime\mod.rs:626:14
  ╰─▶ writing rfc2822 datetime to string should never fail: Error
  help: set the `RUST_BACKTRACE=1` environment variable to display a backtrace.
# NOT OK: panics
```

On this branch 🟢 :
```nu
~> {year: 1900} | into datetime
Mon, 1 Jan 1900 00:00:00 +0200 (in 125 years)
~>  {year: 1000} | into datetime
Wed, 1 Jan 1000 00:00:00 +0200 (1025 years ago)
~> {year: -1000} | into datetime
-1000-01-01T00:00:00+02:00 (3025 years ago)
~> {year: -1000} | into datetime | format date
-1000-01-01T00:00:00+02:00
~> '3000 years ago' | date from-human | format date
-0975-04-11T18:18:24.301641100+02:00
```

# Tests + Formatting

# After Submitting
Nothing required IMO

crates/nu-command/tests/commands/into_datetime.rs

@@ -14,6 +14,16 @@ fn into_datetime_from_record() {
     assert_eq!(expected.out, actual.out);
 }
 
+#[test]
+fn into_datetime_from_record_very_old() {
+    let actual = nu!(r#"{year: -100, timezone: '+02:00'} | into datetime | into record"#);
+    let expected = nu!(
+        r#"{year: -100, month: 1, day: 1, hour: 0, minute: 0, second: 0, millisecond: 0, microsecond: 0, nanosecond: 0, timezone: '+02:00'}"#
+    );
+
+    assert_eq!(expected.out, actual.out);
+}
+
 #[test]
 fn into_datetime_from_record_defaults() {
     let actual = nu!(r#"{year: 2025, timezone: '+02:00'} | into datetime | into record"#);