Rename `Import` resource kind enumeration to `Importer`

[michaeltlombardi]

Summary of the new feature / enhancement

The existing resource kinds are Resource, Adapter, Group, and Import. Of the existing kinds, only Import is a verb instead of a noun. For consistency and to make it simpler to document and discuss, the kind should be renamed from Import to Importer.

Then we can describe how to use and author importer resources that resolve external sources to a set of DSC resource instances.

Proposed technical implementation details (optional)

First, update the Kind enumeration definition from Import to Importer:

https://github.com/PowerShell/DSC/blob/06e79cdabfe6fccdab5dd384387a1a4577843c33/dsc_lib/src/dscresources/resource_manifest.rs#L12-L18

Then, update the reference:

https://github.com/PowerShell/DSC/blob/06e79cdabfe6fccdab5dd384387a1a4577843c33/dsc_lib/src/dscresources/command_resource.rs#L43-L47

Finally, update the resource manifest:

https://github.com/PowerShell/DSC/blob/06e79cdabfe6fccdab5dd384387a1a4577843c33/dsc/include.dsc.resource.json#L6