Teleport Maze Traversal

You are given a 2D grid representing a maze. Each cell in the grid is one of the following characters:

• #: a wall (cannot be traversed)
• .: an open space
• S: the starting position of the maze
• E: the ending (goal) position of the maze
• An uppercase letter (A-Z): represents a teleporter

Each teleporter letter appears exactly twice in the maze. When you step onto a teleporter cell, you are immediately transported to the other cell with the same letter. This teleportation counts as one move. Movement is allowed in the four cardinal directions (up, down, left, right) and stepping into a teleporter cell is considered a valid move.

Your task is to implement a function that, given such a grid, returns the minimum number of moves required to travel from the starting position (S) to the ending position (E). If no valid path exists, the function should return -1.

For example, consider the following grid:

S.A
.#.
A.E

In this grid, stepping onto A in the top row teleports you to A in the bottom row. Taking into account typical moves and teleports, compute the shortest path from S to E.

Note:

• The grid dimensions could be up to 100x100.
• There is exactly one S and one E in the grid.
• There could be multiple teleporter pairs or none at all.

Write a function that solves this problem in an efficient manner.