Require non-whitelisted namespaces to use an alias

[TravisCarden]

• Related: Require certain namespaces to always use the same alias #1617

This is almost (but not quite) a request for the opposite of SlevomatCodingStandard.Namespaces.UseOnlyWhitelistedNamespaces:

I want to require all third-party namespaces to use aliases. To that end, I would like a rule that forces all namespaces to use one, with a configurable whitelist. I could imagine something like this, for example:

<rule ref="SlevomatCodingStandard.Namespaces.RequireNamespaceAliases">
    <properties>
        <property name="namespacesAllowedWithoutAliases" type="array">
            <element value="MyOrg\MyProject"/>
        </property>
    </properties>
</rule>