explicitly disallow using in ambient contexts

[Renegade334]

Since it first supported ERM, the compiler has shunned ambient using: declare [await] using is invalid, and variables appearing in the declaration emit are converted to const.

However, there is a blind spot in the checker, and that is when a variable declaration occurs in already-ambient context:

declare using a: null; // error
declare global {
    using b: null; // not an error
}
namespace N {
    export using c = null; // error
    export declare using d: null; // error
}
declare namespace N {
    using e: null; // not an error
}

Technically this doesn't apply to await using, as the checker will already complain about an invalid await context, but this PR adds errors on both for consistency.

Closes #61752.