Support for "mosaic" puzzle type

[anthonybailey]

This is a very simple puzzle type.

It is almost the same as Minesweeper, but usually presented as a grid shading puzzle.

The difference from Minesweeper is that the cells containing the numeric givens may also be shaded (and all shaded cells in the 3x3 block centered on a given are included in its count.)

Often used to derive pixelated image. The local constraint makes the puzzle less interesting than a nonogram but also makes it better suited to larger images with longer solves.

Documented at e.g. https://www.puzzler.com/puzzles-a-z/mosaic and https://www.puzzle-magazine.com/mosaic-puzzle-strategy.php