[mlir][vector] Add vector.scalable.insert/extract ops

These new operations match the semantics of
llvm.experimental.vector.insert and llvm.experimental.vector.extract.

`vector.scalable.insert` and `vector.scalable.extract` allow,
respectively, insert vectors into scalable vectors, and extract vectors
from scalable vectors.

The discussion about the inclusion of these operations is here:
https://discourse.llvm.org/t/rfc-interfacing-between-fixed-length-and-scalable-vectors-for-vls-vector-code-on-scalable-vector-architectures

Differential Revision: https://reviews.llvm.org/D127875