add prop-dict support for custom directive for mlir-tblgen (#77061)

According to
https://mlir.llvm.org/docs/DefiningDialects/Operations/#custom-directives

,
custom directive supports attr-dict

> attr-dict Directive: NamedAttrList &

But it doesn't support prop-dict which is introduced into MLIR recently.
It's useful to have tblgen support prop-dict like attr-dict. This PR
enable tblgen to support prop-dict

```bash
error: only variables and types may be used as parameters to a custom directive
   ... custom<Print>(prop-dict)
```

Co-authored-by: Fung Xie <ftse@nvidia.com>