Split import into var and type to avoid breaking backword compatability of tsserverlibrary

[mhegazy]

We switched CommandNames from a namespace to a const enum. we have added an import alias declaration to cover that, but since it is a const enum, the import will be elided from the .js file. in #16381 I have removed the const label, this now uncovers the fact that CommandNames do not exisit in the generated JS.

this change switches to use a type/var aliases to ensure it gets emitted in the .js file.

Fix #16393

[ghost]

This works as a quick fix but I think this should be filed as a bug?
Given:

namespace N {
    export const enum E { A, B }
}
namespace M {
    export import E = N.E;
}

I think we should emit something inside M if --preserveConstEnums is on.

We already do that in this case:
a.ts

export const enum E { A, B }

b.ts

export { E } from "./a";

[mhegazy]

we have treated --preserveConstEnums as a debugging flag, it does not change the emitted code, it jsut does not erase the enum declaration, so that at debug time you can look at the reverse map.

we have overloaded that use in the services and the compiler. i wounder if what we need is a flag to inline regular enum values instead.