Allow other ISO 8601 date representations in `date` field

[paulrobertlloyd]

Pages can only include dates in the following formats:

• 2021-01-01
• 2021-01-01 03:10:10
• 2021-01-01 03:10:10 -0700

This appears to follow RFC 3339. However, IS0 8601 dates typically don’t include spaces, but instead use T to separate the date and time. For example, the above can also be represented as:

• 2021-01-01
• 2021-01-01T03:10:10Z
• 2021-01-01Y03:10:10-0700

Support for this representation in Lume would be useful as it would make migrating pages from an Eleventy-powered blog easier. For reference, Eleventy supports thew following date formats:

• any valid YAML date value
• any valid ISO 8601 string that Luxon’s DateTime.fromISO can parse

It should be noted that Luke’s date format filters support both of the above representations (ATOM and DATETIME).

ISO 8601 dates also don’t need to include - or : separators, so the following dates are also valid:

• 20210101
• 20210101T031010Z
• 20210101Y031010-0700

There may be less need to support this, although I’be come across it on my own site where I’m parsing iCalendar data which formats dates using this representation. As I can’t use them directly when generating pages, I need to manipulate them to the format Lume allows.

[oscarotero]

Deno' std modules include some datetime utils, like this parser; https://deno.land/std@0.157.0/datetime/mod.ts?s=parse but it doesn't auto-detect the format. Maybe the date-fns library (a already used dependency) can handle these formats.

I like also the git related dates of Eleventy. It's something that @dz4k implemented in his site: https://github.com/dz4k/denizaksimsek.com/blob/master/_build/get-dates-from-git.ts