[mlir][linalg][python] Adapt the OpDSL to use scalars.

The patch replaces the existing capture functionality by scalar operands that have been introduced by https://reviews.llvm.org/D104109. Scalar operands behave as tensor operands except for the fact that they are not indexed. As a result ScalarDefs can be accessed directly as no indexing expression is needed.

The patch only updates the OpDSL. The C++ side is updated by a follow up patch.

Differential Revision: https://reviews.llvm.org/D104220