config: Search for authentication token defined by environment variable

As discussed on npm.community[1], the fact that
npm registry authentication tokens
cannot be defined using environment variables
does not seem justified anymore.

The restriction is caused by the config loader translating
* all `_` to `-`
* the whole variable name to lowercase
while the credential checker expects a key ending in `:_authToken`.

This change fixes the problem
by having the credential checker try
a key ending in `:-authtoken` after it tried `:_authToken`.

Closes npm/npm#15565

[1]: https://npm.community/t/cannot-set-npm-config-keys-containing-underscores-registry-auth-tokens-for-example-via-npm-config-environment-variables/233

lib/config/get-credentials-by-uri.js

@@ -34,6 +34,12 @@ function getCredentialsByURI (uri) {
     return c
   }
 
+  if (this.get(nerfed + ':-authtoken')) {
+    c.token = this.get(nerfed + ':-authtoken')
+    // the bearer token is enough, don't confuse things
+    return c
+  }
+
   // Handle the old-style _auth=<base64> style for the default
   // registry, if set.
   var authDef = this.get('_auth')