Add more assert cases in the codebase (#1115)

* Add assert for FACING_GRASS_1 and FACING_GRASS_2

* Add more assert cases

* Add assert for HP bar colors

* Assert LINK_NULL == 0

* Fix assert syntax

* Add asserts for WILD_BATTLE == 1

* Add additional WILD assert

- I need to sleep more...

* Revert asserts for WILD_BATTLE and LINK_NULL

* Commit suggested changes

* Add more assert cases

* Fix syntax error

engine/pokemon/european_mail.asm

@@ -6,16 +6,20 @@ ParseMailLanguage:
 	cp "E"
 	ret nz
 	ld a, [hli]
-	inc c ; MAIL_LANG_FRENCH
+	assert MAIL_LANG_ENGLISH + 1 == MAIL_LANG_FRENCH
+	inc c
 	cp "F"
 	ret z
-	inc c ; MAIL_LANG_GERMAN
+	assert MAIL_LANG_FRENCH + 1 == MAIL_LANG_GERMAN
+	inc c
 	cp "G"
 	ret z
-	inc c ; MAIL_LANG_ITALIAN
+	assert MAIL_LANG_GERMAN + 1 == MAIL_LANG_ITALIAN
+	inc c
 	cp "I"
 	ret z
-	inc c ; MAIL_LANG_SPANISH
+	assert MAIL_LANG_ITALIAN + 1 == MAIL_LANG_SPANISH
+	inc c
 	cp "S"
 	ret z
 	ld c, MAIL_LANG_ENGLISH