Interpret `mlir__constant.dense` op

`Veir/Interpreter/Basic.lean`'s `Llvm.interpretOp` currently executes `mlir__constant.dense` as `none`. Interpret this correctly. It should return the value of the constant (in some suitably packed form).

Start in Veir/Interpreter/Basic.lean and read Llvm.interpretOp, especially the existing handling of mlir__constant.dense. Determine how constant values are represented or packed by nearby interpreter cases, then implement the operation so it returns the constant value instead of none. Done means the operation produces a suitably packed constant result.

Written by the indexing model from the issue text.