Splitting sitemaps into multiple files

[kaspars-ozols]

There is a constraint defined in sitemap specs - a single sitemap file should not contain more than 50,000 URLs and must be no larger than 50MB. The package seems to ignore this and always generate a single sitemap file.

This becomes a problem in sites with a lot of content / large catalogs. Should implement sitemap splitting in accordance with the spec:

https://www.sitemaps.org/protocol.html#index