[mlir][sparse] add affine parsing to new surface syntax for STEA

[mlir][sparse] add affine parsing to new surface syntax for STEA

(1) uses the previously introduce API to reuse AffineExpr parser without codedup
(2) solves the look-ahead problem when parsing level spec

Reviewed By: Peiming

Differential Revision: https://reviews.llvm.org/D154254