Initial project skeleton: date (#11522)

This PR proposes integrating `date` into OSSFuzz. 

I believe that this repository is security-relevant due to its high
star-count and its being a dependency in various large projects, some of
which are under OSSFuzz already (such as arrow). This library also
parses user-input in order to convert date strings into various formats,
thus exposing itself to potential exploitation.

Thank you for your review!

projects/date/project.yaml

@@ -0,0 +1,4 @@
+homepage: "https://howardhinnant.github.io/date/date.html"
+language: c++
+primary_contact: "[email protected]"
+main_repo: "https://github.com/HowardHinnant/date"