Create and use an `nginx` type now that we have a specific versioning scheme for it

[Hritik14]

We should create and use an nginx type now that we have a specific versioning scheme for it
https://github.com/package-url/purl-spec/pull/139/files#diff-dda3c48de6da2f63bfa5902e41755b27a2f1d8c186a21dcc2641c42d4f5cfaebR295
This should be tracked in an issue... and need to be done at the purl spec type

Originally posted by @pombredanne in #525 (comment)