Groundwork for Galarian Bird event

This adds the first half of an event where you will find the Galarian Birds. It is not finished yet.

It currently bugs out when Galarian Zapdos tries to run away and I don't know why. Frame-by-frame, it seems to be when the player has to move. I've tried different things to make this less bad, but they don't work.

Here's how it's meant to go:
1) Articuno flies away (done)
2) Zapdos barges past the player like Silver does in GSC (bugs out)
3) Moltres moves in front of the player and engages (not done)

If anyone has a better way to do this, please let me know!

Minor changes;
- Connection for Brunswick-Citrine looks nicer
- Celeste has a few changes to make the event run easier.

constants/event_constants.asm

@@ -788,6 +788,7 @@
 	const_next $880
 	const EVENT_GOT_HM03
 	const EVENT_GOT_MELTAN
+	const EVENT_BIRDS_FOUND ; hehe
 
 ; Cerulean Cave events
 	const_next $8C0

constants/hide_show_constants.asm

@@ -35,9 +35,9 @@ DEF SHOW EQU $15
 	const HS_SAFFRON_CITY_E                ; 17
 	const HS_SAFFRON_CITY_F                ; 18
 	const HS_SAFFRON_CITY_UP_GRADE         ; 19
-	const HS_ROUTE_9_ITEM                  ; 1C X
+	const HS_CELESTE_ARTICUNO_G            ; 1C was unused HS_ROUTE_9_ITEM
 	const HS_ROUTE_12_SNORLAX              ; 1D
-	const HS_ROUTE_12_ITEM_1               ; 1E X
+	const HS_CELESTE_ZAPDOS_G              ; 1E X was unused HS_ROUTE_12_ITEM_1
 	const HS_ROUTE_12_ITEM_2               ; 1F X
 	const HS_ROUTE_15_ITEM                 ; 20 X
 	const HS_ROUTE_16_SNORLAX              ; 21