Extract `game::find_stairs` from `game::find_or_make_stairs` #94610

Job	Run time
matrix-variables	1s
skip-duplicates-code	4s
skip-duplicates-data	5s
Basic Build and Test (Clang 10, Ubuntu, Curses)	1h 5m 12s
Clang 14, macOS 12, Tiles, Sound, x64 and arm64 Universal Binary	6m 30s
GCC 9, Curses, LTO	1s
Clang 12, Ubuntu, Tiles, ASan	1s
GCC 11, Ubuntu, Curses, ASan	1s
GCC 12, Ubuntu cross-compile to MinGW-Win64, Tiles, Sound	1s
GCC 9, Ubuntu, Tiles, Sound, CMake	1s
emscripten	0s
	1h 11m 57s

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