Adds a message that appears in an empty databases view

Also, fixes a regex.

extensions/ql-vscode/package.json

@@ -636,6 +636,10 @@
       {
         "view": "codeQLQueryHistory",
         "contents": "Run the 'CodeQL: Run Query' command on a QL query.\n[Run Query](command:codeQL.runQuery)"
+      },
+      {
+        "view": "codeQLDatabases",
+        "contents": "Add a Code QL database:\n[From a folder](command:codeQLDatabases.chooseDatabaseFolder)\n[From an archive](command:codeQLDatabases.chooseDatabaseArchive)\n[From a URL (as a zip file)](command:codeQLDatabases.chooseDatabaseInternet)\n[From LGTM](command:codeQLDatabases.chooseDatabaseLgtm)"
       }
     ]
   },

extensions/ql-vscode/src/databases-ui.ts

@@ -611,7 +611,8 @@ export class DatabaseUI extends DisposableObject {
   }
 }
 
-const dbRegeEx = /^db-(javascript|go|cpp|java|python)$/;
+// TODO: Get the list of supported languages from a list that will be auto-updated.
+const dbRegeEx = /^db-(javascript|go|cpp|java|python|csharp)$/;
 function isLikelyDbFolder(dbPath: string) {
   return path.basename(dbPath).match(dbRegeEx);
 }