Split import into var and type to avoid breaking backword compatability of tsserverlibrary (#16409)

Author: mhegazy

src/server/session.ts

@@ -112,7 +112,13 @@ namespace ts.server {
         return true;
     }
 
-    export import CommandNames = protocol.CommandTypes;
+    // CommandNames used to be exposed before TS 2.4 as a namespace
+    // In TS 2.4 we switched to an enum, keep this for backward compatibility
+    // The var assignment ensures that even though CommandTypes are a const enum
+    // we want to ensure the value is maintained in the out since the file is
+    // built using --preseveConstEnum.
+    export type CommandNames = protocol.CommandTypes;
+    export const CommandNames = (<any>protocol).CommandTypes;
 
     export function formatMessage<T extends protocol.Message>(msg: T, logger: server.Logger, byteLength: (s: string, encoding: string) => number, newLine: string): string {
         const verboseLogging = logger.hasLevel(LogLevel.verbose);