fix: slugFormatter populates date template variables using date from entry if it exists

[fionaRowan]

related to issue #6801 , though takes a different approach than that requested in the issue description. this PR tentatively implements solution 1 outlined in this issue comment, though could pivot to another solution if preferred!

Summary

currently, when configuring the slug, the template tags {{year}}, {{month}}, etc are always pulled from the hardcoded "now".

in this PR, if the entry contains an inferred 'date' field, we use that value when generating the slug.

thus, the following slug config would pull the template tags {{year}}, {{month}}, etc from the entry's front matter inferred 'date' field if it exists (or otherwise, "now"):

slug: "{{year}}-{{month}}-{{day}}-{{slug}}" 

notes for evaluating slugFormatter's expected behavior w.r.t. date template tags:

• this PR makes the slugFormatter's behavior consistent with that of folderFormatter, which already uses the date from the entry's front matter by default.
• this PR aligns the slugFormatter's behavior with this piece of documented behavior:

| slug | Template for filenames. {{year}}, {{month}}, and {{day}} pulls from the post's date field or save date. {{slug}} is a URL-safe version of the post's title. Default is simply {{slug}}. |

• however, this PR makes the slugFormatter's behavior more nuanced than this piece of documented behavior implies

{{year}}: 4-digit year of the file creation date

Test plan

• wrote a unit test which I verified passes
• (still working on this) built the library + pointed my own static generator site to that library locally to verify that the slug template variables {{year}}, {{month}}, etc are pulled from the entry's front matter "date" field if it exists

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[martinjagodic]

@fionaRowan this is a very well-written PR!

I like the idea of your second point: allow the user to configure which date to use. For instance, Hugo also uses publishDate, and it would be helpful to have that choice.

But, how should the user pass that field?

option 1:

slug: "{{fields.date.year}}-{{fields.date.month}}-{{fields.date.day}}-{{slug}}"

The original proposal from #6801. It feels too verbose, and .year sounds like a field, not a modifier.

option 2:

slug: "{{fields.date | date("YYYY-MM-DD)}}-{{slug}}"

This matches your option 3, I like it better. If I understand correctly, this is possible if we merge #6690?

In any case, I would first merge what you did here, because it's a well-rounded PR. Just let me know once you complete the testing, and please share your thoughts on how to continue.