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:

declareusing a: null;// error declare global { using b: null;// not an error}namespaceN{export using c=null;// error export declareusingd: null;// error} declare namespaceN{ 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.