disallow non-global foreign import of variables on wasm

[laytan]

AFAIK (checked with clang, which disallows this) this is not allowed, + we are currently ignoring the module for variables.

foreign import foo "foo"

foreign foo {
   bar: baz,
}

should not be allowed, but (global)

foreign {
    bar: baz,
}

is allowed just going of what clang and LLVM allow