Add support for parsing namespaces without "name" fields

[bteng22]

Context

My team uses packageurl-js and needs to handle use cases where we're creating configuration options to deny all packages under a specific namespace. We currently use packageurl-js to parse all of our PURLs (including namespaces i.e. pkg:maven/com.fasterxml.jackson.core/) and it'd be helpful to have an interface that will allow us use the same parser for these options.

Current Behavior

The current packageurl-js library expects a "name" field to be present in the package URL.

Desired Behavior

We would like to be able to create and parse package URLs that have a "namespace" field, but no "name" field.