[DNM][DNM][WIP] Projectile backend code port and update

Ports#54924,  #65061, #59804 from /tg.
adds auxiliary code from /tg to make code work.

code/__DEFINES/_flags/_flags.dm

@@ -104,6 +104,7 @@ GLOBAL_LIST_INIT(bitflags, list(
 #define PASSBLOB		(1<<3)
 #define PASSMOB			(1<<4)
 #define PASSCLOSEDTURF	(1<<5)
+/// Let thrown things past us. **ONLY MEANINGFUL ON pass_flags_self!**
 #define LETPASSTHROW	(1<<6)
 
 //Movement Types
@@ -111,8 +112,8 @@ GLOBAL_LIST_INIT(bitflags, list(
 #define FLYING				(1<<1)
 #define VENTCRAWLING		(1<<2)
 #define FLOATING			(1<<3)
-///When moving, will Bump()/Cross()/Uncross() everything, but won't be stopped.
-#define UNSTOPPABLE			(1<<4)
+///When moving, will Bump()/Cross()/Uncross() everything, but won't stop or Bump() anything.
+#define PHASING				(1<<4)
 ///Applied if you're crawling around on the ground/resting.
 #define CRAWLING			(1<<5)
 

code/__DEFINES/projectiles.dm

@@ -1,3 +1,14 @@
+
+// check_pierce() return values
+/// Default behavior: hit and delete self
+#define PROJECTILE_PIERCE_NONE 0
+/// Hit the thing but go through without deleting. Causes on_hit to be called with pierced = TRUE
+#define PROJECTILE_PIERCE_HIT 1
+/// Entirely phase through the thing without ever hitting.
+#define PROJECTILE_PIERCE_PHASE 2
+// Delete self without hitting
+#define PROJECTILE_DELETE_WITHOUT_HITTING 3
+
 /// This atom should be ricocheted off of from its inherent properties using standard % chance handling.
 #define PROJECTILE_RICOCHET_YES 1
 /// This atom should not be ricocheted off of from its inherent properties.