fix: 🐛 use correct formatting for section titles in exercise detail view

[SpacyRicochet]

Introduces a relativeString method as a Date extension to perform the necessary formatting

Fixes the headers in the ExerciseDetailView to display correct relative strings for the dates

Adds more set entries to the preview model container for exercises

Specifies two previews for ExerciseDetailView, one for an empty view and one for a view with entries

closes: #36

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Notes

After some investigation, I couldn't figure out how to use reference(to:allowedFields:maxFieldCount:thresholdField:) properly, especially since it looks like a very new API which is horribly documented.

Instead, I opted to for a combination of formatters, depending on the difference between now and the section date;

1. If the section date is today, we specify "Today" to be used. I opted for a localized string here, because the standard relative formatting uses "Now", which doesn't fit as well.
2. If the section date is three days or less away, we use relative formatting. The three days is arbitrary, but it should be good enough for our purposes.
3. After that, we show the date without the year. The year is obvious information which doesn't need to be repeated.
4. Unless it's actually a different year, where it starts making sense to also show the year.

[SpacyRicochet]

@heyjaywilson I see that the PR title validation fails, but I can't figure out why exactly. Do you know?

[heyjaywilson]

@SpacyRicochet I adjusted to not include the issue number in the title. That's just a personal preference. I'd rather not have it in the title and close the issue via description.

The title needs to follow conventional commit formatting. So your first title didn't do that. It was Fix section titles... where it should be fix: section titles. Other than that, I have an action that adds the shortcode for the emoji.