feat(data/finset/basic): add card_insert_eq_ite (#10209)

Adds the lemma `card_insert_eq_ite` combining the functionality of `card_insert_of_not_mem` and `card_insert_of_mem`, analogous to how `card_erase_eq_ite`.

src/data/finset/basic.lean

@@ -2293,6 +2293,16 @@ theorem card_insert_le [decidable_eq α] (a : α) (s : finset α) : card (insert
 by by_cases a ∈ s; [{rw [insert_eq_of_mem h], apply nat.le_add_right},
 rw [card_insert_of_not_mem h]]
 
+/-- If `a ∈ s` is known, see also `finset.card_insert_of_mem` and
+`finset.card_insert_of_not_mem`. -/
+theorem card_insert_eq_ite [decidable_eq α] {a : α} {s : finset α} :
+  card (insert a s) = if a ∈ s then card s else card s + 1 :=
+begin
+  by_cases h : a ∈ s,
+  { rw [card_insert_of_mem h, if_pos h] },
+  { rw [card_insert_of_not_mem h, if_neg h] },
+end
+
 @[simp] theorem card_singleton (a : α) : card ({a} : finset α) = 1 := card_singleton _
 
 lemma card_singleton_inter [decidable_eq α] {x : α} {s : finset α} : ({x} ∩ s).card ≤ 1 :=