Set skillFlag from gem stat map (#9471)

Currently we manually set a number of flags for things like area damage and projectile damage for skills but a lot of these can be automated if we just map a particular stat on the gem

In this case I did it for arrows on skills as there are many skills that fire arrows and manually adding the flag to every skill is laborious and error prone
Many skills fire arrows which are projectiles, but not all projectiles are arrows so in the case of Thunderburst the secondary skill fire projectiles and shouldn't benefit from additional arrow mods like on Death's Harp

Co-authored-by: LocalIdentity <localidentity2@gmail.com>

Author: LocalIdentity

src/Data/Global.lua

@@ -139,6 +139,7 @@ KeywordFlag.Lightning =	0x00000080
 KeywordFlag.Chaos =		0x00000100
 KeywordFlag.Vaal =		0x00000200
 KeywordFlag.Bow =		0x00000400
+KeywordFlag.Arrow =		0x00000800
 -- Skill types
 KeywordFlag.Trap =		0x00001000
 KeywordFlag.Mine =		0x00002000