[Feature] Support multiple symbols in a single cell

[T0nyX1ang]

The solver should support puzzles with multiple symbols is a cell.

The example is a Kropki puzzle: https://puzz.link/p?kropki/8/8/123226001op40pa160a0071002309759c0314i

[T0nyX1ang]

The current plan is to move the category key from the last part of a symbol string to the dictionary key, just as the style of edge and line encodings.

The encoding will be converted from:

    symbol: Dict[Tuple[int, int]]

to:

    symbol: Dict[Tuple[int, int, Direction]]