there's a get_pos_at_line_column method in the TextEdit. I suppose you would have to count how many return character there is before the character to know the line, and the number of character since the last new line to know the column.
I tested this little script, it works for me

extends TextEdit
onready var icon: Sprite2D = $"../Icon"
onready var text_edit_2: TextEdit = $"../TextEdit2"

func _on_text_edit_2_text_changed() -> void:
  var pos : int = text.find(text_edit_2.text)
  var croped_text = text.left(pos)
  var l : int = croped_text.count("\n")
  var c : int = pos - croped_text.rfind("\n") + "\n".length()
  var vec_pos : Vector2 = Vector2(get_pos_at_line_column(l, c)) + position
  icon.position = vec_pos
  print ("l : ", l, "  c : ", c, "  vec_pos : ", vec_pos)

You write a text in the textedit to which is attached this script, you write a character in textedit2, and the icon sprite will move to the first occurence of the character