Extend `--convert-tensor-to-scalars` beyond `tensor.insert/extract`

The `--convert-tensor-to-scalars` pass introduced in PR #763 currently only handles `tensor.insert` and `tensor.extract` operations, but not other similar tensor operations, such as `tensor.slice`.

As part of this, it'd be a good idea to extend the tests for this pass beyond the tests generated from HEIR's polynomial-pipeline to a wider range of general `tensor` use cases. (Especially since we might want to suggest #524 and this pass for inclusion upstream at some point),

(labelling as `tensor_ext`, as it's tensor-related, even though this is technically a transform for "raw" `tensor` ops rather than `tensor_ext` ops)