[mlir][transform] Add PackedOrDynamicIndexList helper

This customer parser/printer is similar to DynamicIndexList, but has special syntax for the case where one handle represents the entire list.

Example:
```
// Regular index list
[10, 20, %val]

// Packed handle (no square parentheses)
%val
```

Differential Revision: https://reviews.llvm.org/D138825