Only parse safe as contextual kw in extern blocks

I don't like the party of `bool`s that is becoming, but two isn't worth a refactoring yet IMO.

crates/parser/src/grammar/expressions.rs

@@ -66,7 +66,7 @@ pub(super) fn stmt(p: &mut Parser<'_>, semicolon: Semicolon) {
 
     // test block_items
     // fn a() { fn b() {} }
-    let m = match items::opt_item(p, m) {
+    let m = match items::opt_item(p, m, false) {
         Ok(()) => return,
         Err(m) => m,
     };