make `Parser::parse_foreign_item()` return a foreign item or error
Fixes `Parser::parse_foreign_item()` to follow the convention of `parse_trait_item()` and `parse_impl_item()` in that it *must* parse an item or return an error, and then the caller is responsible for detecting the closing delimiter.
This prevents it from looping endlessly on an unexpected token in `ext/expand.rs` where it was also leaking memory by continually pushing to `Parser::expected_tokens` via `Parser::check_keyword()`.
closes#54441
r? @petrochenkov
cc @dtolnay
a95a6e287a