Use techie offset for basic ISO format

[snowwwz]

Description of the Bug

with format: 'basic' option, toISO and toISOTime is supposed to return techie offset (ex. +0900)

luxon/src/datetime.js

Line 1825 in 3e9983c

* @example DateTime.now().toISO({ format: 'basic' }) //=> '20170422T204705.335-0400'

but, it returns short offset (ex. +09:00) with : instead.

% node
Welcome to Node.js v22.3.0.
Type ".help" for more information.
> const { DateTime } = await import("luxon");
undefined
> DateTime.now().toISO({ format: 'basic' });
'20241003T231708.609+09:00'

[linux-foundation-easycla]

• ✅login: snowwwz / (7cfa0fa)

The committers listed above are authorized under a signed CLA.

[mceachen]

Why would you make this change? It breaks prior default behavior and will almost certainly cause downstream issues.

Technically, ISO8601 states that the colon is optional, the default ISO format includes the colon. See https://en.wikipedia.org/wiki/ISO_8601