fix malformed parse tree for invalid let struct pattern (#7782)

Fixes the malformed parse tree produced for an invalid let struct
pattern containing a single identifier (e.g., `let {s};`).

As pointed out by @DavidLoftus, the parser should produce a parse tree
similar to that of `let {ref s};`, since both are missing a binding
power operator `:`, and both do not have a `.` preceding the identifier
(i.e., `state.in_field_shorthand_pattern == true`).

This means that the parser can produce an the `InvalidParse` node just
as it does for `let {ref s};`.

Closes #7674

toolchain/parse/handle_pattern.cpp

@@ -47,9 +47,10 @@ auto HandlePattern(Context& context) -> void {
           state.binding_context, state.ambient_precedence);
       break;
     default:
-      if (context.PositionKind().is_word() &&
-          context.PositionKind(Lookahead::NextToken)
-              .is_binding_pattern_operator()) {
+      if ((context.PositionKind().is_word() &&
+           context.PositionKind(Lookahead::NextToken)
+               .is_binding_pattern_operator()) ||
+          state.in_field_shorthand_pattern) {
         context.PushStateForPattern(
             StateKind::BindingPattern, state.in_var_pattern,
             state.in_unused_pattern, state.in_field_shorthand_pattern,