Function generate_variable 

pub fn generate_variable(
    decl: &VariableDecl,
    ctx: &ModuleContext,
    cgctx: Option<&CodegenContext<'_>>,
    doc: &Option<String>,
    js_namespace: Option<&str>,
    scope: ScopeId,
) -> TokenStream

Generate a wasm_bindgen extern block for a global constant/variable.