feat(c_parser): support C PtrDecl

[yxliang01]

The C PtrDecl node is unsupported. While it might be difficult to add pointers to the Clara's internal representation, I think it would be still beneficial to add PtrDecl support as it will require less effort to input a C file having unused pointers.

[iradicek]

The problem is not internal representation. but matching & repair algorithms. It's non-trivial to extend those to support mutable state.

To properly load such a file, one would need to load such a file and then remove all unused pointer (to make sure they are all really unused).

This is definitively possible, but it would be interesting to have some test cases.