Add isoyear

[MichaelChirico]

LGTM, I only question if we should also add isoyear = function(x) as.integer(format(as.IDate(x), "%G"))?

Since isoyear("2019-12-30") is 2020.

Originally posted by @ben-schwen in #7144 (comment)

This is a natural complement to isoweek(). As with isoweek(), we can quickly include a fast {base}-wrapped implementation, and try and improve it later.

Related: #2611.

[venom1204]

Hi @MichaelChirico, @tdhock

I would like to work on resolving this issue.

My plan is to submit a pull request that includes the following changes:

• Add the isoyear = function(x) as.integer(format(as.IDate(x), "%G")) function to R/IDateTime.R.
• Add tests to cover the new function, including the key edge cases (like isoyear("2019-12-30") returning 2020).
• Update the documentation and NAMESPACE file so the function is available and documented for users.
• Add a note about the new feature to NEWS.md.

if this sounds good i will be happy to submit a pr.

[MichaelChirico]

yes go ahead