feat: finish? produces finish only suggestion

[leodemoura]

This PR adds a new suggestion to finish?. It now generates the grind tactic script as before, and a finish only tactic. Example:

/--
info: Try these:
  [apply] ⏎
    instantiate only [findIdx, insert, = mem_indices_of_mem]
    instantiate only [= getElem?_neg, = getElem?_pos]
    cases #1bba
    · instantiate only [findIdx]
    · instantiate only
      instantiate only [= HashMap.mem_insert, = HashMap.getElem_insert]
  [apply] finish only [findIdx, insert, = mem_indices_of_mem, = getElem?_neg, = getElem?_pos, = HashMap.mem_insert,
    = HashMap.getElem_insert, #1bba]
-/
example (m : IndexMap α β) (a : α) (b : β) :
    (m.insert a b).findIdx a = if h : a ∈ m then m.findIdx a else m.size := by
  grind => finish?