Introduce before and after filters

[ccjmne]

• Are you doing the PR on the next branch?
• Have you created/updated the relevant documentation page(s)?

Fixes #2619.

• The "before" filter is exclusive, and the "after" one is inclusive; it
  may be better thought of as a "since" than a strict "after".
• Pages with no dates are never included.
• Dates can be 8601 default, like: 2023-12-01T12:00:00Z
  or only have the date component, like: 2023-12-01

[ccjmne]

With before being exclusive and after being inclusive, we have effectively access to the >= and < comparisons.

I figured it would be somewhat useful when honing in on a particular month or particular year:

• this month's articles: section.pages | after(date="2025-11-01")
• last year's articles: section.pages | after(date="2024-01-01") | before(date="2025-01-01")

[Keats]

How about adding inclusive as param to both, defaulting to false so people can choose?

[Keats]

looking at the usage, I'm wondering if it would be better to have the before/after work on a date rather than Page[]. It would be usable in more places

[ccjmne]

Yeah, it's a bit peculiar to have that filter work on an array of items that have a date property, that's pretty arbitrary.

To clarify you suggest we have some after and before filters that yield either true or false when they're used on a date, that we'd use like:

{% for page in section.pages %}
    {% if page.date | before(date="2025-01-01") %} {% continue %} {% endif %}
    {{ page.content | markdown | safe }}
{% endfor %}

Correct?
If so, that makes sense as well, I can do that :)

[ccjmne]

I don't suppose it'd make sense to have it run on BOTH OF Page[]/Section[] and date, right? It'd be somewhat neat to have a filter that directly eliminates stuff from an array, I quite like the section.pages | after(xxx) construct.

But supporting both if page.date | before(xxx) and for page in section.page | before(xxx) is semantically overloaded, I believe.
It's very, very easy to implement and maintain, however ;)

I'll go ahead and assume you'd rather want only the:

{% if page.date | before(date="2025-01-01") %}

We can always revert that commit later anyway.

[Keats]

I think it's better to work on a date rather than on a page/section. I think for that it will be better to design an improved filter filter for tera2 so you can apply a filter on every elements of a collection

[ccjmne]

Oh hell yeah, a filter filter would be awesome :)

In the meantime, I went ahead and made the adjustments you requested. This approach will definitely be perfect once the filter filter is available, and it's already still quite useable.

Let me know how you want to proceed from here!

[ccjmne]

How about adding inclusive as param to both, defaulting to false so people can choose?

Ha! Yeah, seems obvious in retrospect. I'll do just that (tonight).

[ccjmne]

Hey @Keats, I addressed your comments, do you think it makes sense to merge this as it now is?

[Keats]

I've just realised, these shouldn't be filters but tests

[ccjmne]

Ah, like xxx is defined? Is defined one such "test"?
I could look into having them be that, that does indeed make more sense :)

[Keats]

Yes exactly, it's basically a shorthand for a function returning a bool