Add support for accepting directories to sln add and remove commands.

This commit adds support for specifying directories containing a single
project to both the `sln add` and `sln remove` commands.

Additionally, the output from `sln remove` has been improved to not
mention "project references".

Fixes issue #7343.

src/dotnet/xlf/CommonLocalizableStrings.es.xlf

@@ -674,6 +674,11 @@
         <target state="translated">No realiza una restauración implícita al ejecutar el comando.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ProjectRemovedFromTheSolution">
+        <source>Project `{0}` removed from the solution.</source>
+        <target state="new">Project `{0}` removed from the solution.</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>