refactor: extract accessibility code into dedicated modules

Add Source/accessibility/ with four new modules:

- speech.cpp/hpp: low-HP warnings, durability warnings, boss/monster
  HP announcements, interactable door announcements
- tracker.cpp/hpp: TrackerTargetCategory, path-finding predicates,
  tracker navigation (cycle, navigate, auto-walk)
- town_navigation.cpp/hpp: town NPC selection, listing, auto-walk
- location_speech.cpp/hpp: BFS path-finding for speech, player status
  announcements, nearest-exit/stairs/portal speech, keyboard walk

Wire the new modules into diablo.cpp via four include lines.
Declare CanPlayerTakeAction() and CancelAutoWalk() in diablo.h.
Add the four new source files to Source/CMakeLists.txt.

Author: yuripourre

Source/CMakeLists.txt

@@ -3,6 +3,10 @@ include(functions/devilutionx_library)
 include(functions/genex)
 
 set(libdevilutionx_SRCS
+  accessibility/location_speech.cpp
+  accessibility/speech.cpp
+  accessibility/town_navigation.cpp
+  accessibility/tracker.cpp
   appfat.cpp
   automap.cpp
   capture.cpp