Add triangle and diagonal specifiers based on std::linalg

[BenBrock]

Add specifiers triangle and diagonal specifiers for triangular operations. These are directly based on the tag classes from std::linalg [link].

Implementations should statically inspect the types of the triangle and diagonal tags in a manner similar to the following example (again directly borrowed from std::linalg):

template<class Triangle>
void triangular_matrix_vector_2x2_product(
       mdspan<const float, extents<int, 2, 2>> m,
       Triangle t,
       mdspan<const float, extents<int, 2>> x,
       mdspan<float, extents<int, 2>> y) {

  static_assert(is_same_v<Triangle, lower_triangle_t> ||
                is_same_v<Triangle, upper_triangle_t>);

  if constexpr (is_same_v<Triangle, lower_triangle_t>) {
    y[0] = m[0,0] * x[0]; // + 0 * x[1]
    y[1] = m[1,0] * x[0] + m[1,1] * x[1];
  } else { // upper_triangle_t
    y[0] = m[0,0] * x[0] + m[0,1] * x[1];
    y[1] = /* 0 * x[0] + */ m[1,1] * x[1];
  }
}

[spencerpatty]

These look great and mirroring std::linalg is the right choice, Approved once CI completes