feat: add support for field date

Great tool! But as far as I can see the field date is not considered for sorting etc. It doesn't matter whether date={YYYY} or date={YYYY-MM-DD} it is always omitted.

[monperrus]

Does this answer your question? https://github.com/monperrus/bibtexbrowser/blob/master/bibtexbrowser-documentation.wiki#How_to_configure_the_sorting_order_of_Bibtex_entries

Honestly, it doesn't. I am using the WP-Plugin and I barely have a clue how to change things in PHP.
I was wondering if it is possible that the (rather complex) field date is considered automatically if there is no field year.

[monperrus]

if it is possible that the (rather complex) field date is considered automatically if there is no field year.

That's a good idea, pull request welcome!

Until someone can come up with a pull request, there is a solution outside of bibtexbrowser by converting date into year, month, day: https://tex.stackexchange.com/a/265979/98739 (see the discussion in plk/biber#301)