Add npm virtual repositories

docs/npm/virtual.md

@@ -0,0 +1,30 @@
+# `npm` virtual
+
+An npm virtual serves one registry over local and remote npm members:
+`npm install` and `npm view` work against `/api/v1/virtual/<name>/`.
+
+- Versions are the union across members; on a duplicate version the earlier
+  member wins.
+- Each dist-tag comes from the first member defining it, and is dropped if it
+  names a version missing from the merged set.
+- Tarball URLs point at the virtual (`<member>/<path>`), which 302s to the
+  owning member.
+- Full and abbreviated (`application/vnd.npm.install-v1+json`) packuments are
+  both served.
+- A member error fails the request with 502; a member without the package is
+  skipped. Merges are cached for 60s per package.
+
+```hcl
+resource "artifactapi_virtual" "npm" {
+  name         = "npm"
+  package_type = "npm"
+  members      = [artifactapi_local_npm.internal.name, artifactapi_remote_npm.npmjs.name]
+}
+```
+
+```ini
+# .npmrc
+registry=https://artifactapi.example.com/api/v1/virtual/npm/
+```
+
+Publish to the local member directly; the virtual is read-only.