mathlib3

8ff51ea9 - feat(analysis/inner_product_space/pi_L2): norms of basis vectors (#19020)

feat(analysis/inner_product_space/pi_L2): norms of basis vectors (#19020) This adds `‖euclidean_space.single i (a : 𝕜)‖ = ‖a‖` and other similar results. They hold more generally for `pi_Lp`, so they are proven there first. The statement of `linear_isometry_equiv.pi_Lp_congr_left_single` has also been corrected to include the missing typecast.