Removed SDL_bool in favor of plain bool

We require stdbool.h in the build environment, so we might as well use the plain bool type.

If your environment doesn't have stdbool.h, this simple replacement will suffice:
typedef signed char bool;

include/SDL3/SDL_assert.h

@@ -220,7 +220,7 @@ typedef enum SDL_AssertState
  */
 typedef struct SDL_AssertData
 {
-    SDL_bool always_ignore;  /**< SDL_TRUE if app should always continue when assertion is triggered. */
+    bool always_ignore;  /**< true if app should always continue when assertion is triggered. */
     unsigned int trigger_count; /**< Number of times this assertion has been triggered. */
     const char *condition;  /**< A string of this assert's test code. */
     const char *filename;  /**< The source file where this assert lives. */