Unable to parse designated initialisers

The following piece of code is parsed as water:
```c
int arr[5] = {
  [0] = 12,
  [1] = 34,
};
```

The grammar does not currently support designated initialisers. More details about designated initialisers and the various forms they can take may be found [here](https://gcc.gnu.org/onlinedocs/gcc/Designated-Inits.html).