EDTF dates -- ranges ("2021/2026"), uncertain/approximate dates
("2004-06~"), and BCE dates ("-0429") -- and common equivalent notations
("1995-1996", "2021-22", "~1995", "ca. 1995", "429 BCE") are now passed
to citeproc-js as CSL date ranges, circa flags, and negative years.
Previously, such dates were mangled or dropped entirely unless entered as
CSL variables in Extra.
CSL date variables in Extra get the same parsing.
Other date handling in the client doesn't understand EDTF yet: the y/m/d
indicator in the date field doesn't reflect EDTF parsing, date searches
only match a range by its start date, and BCE dates still can't be
sorted.
Addresses #637
