Model var as a pattern operator (#4720)

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/check/handle_impl.cpp

@@ -42,6 +42,7 @@ auto HandleParseNode(Context& context, Parse::ImplIntroducerId node_id)
   // TODO: Instead use a separate parse node kinds for `impl` and `impl forall`,
   // and only push a pattern block in `forall` case.
   context.pattern_block_stack().Push();
+  context.full_pattern_stack().PushFullPattern();
   return true;
 }