fix: use foreach instead for in array append!

[prashantrahul141]

fixes #55063

Use foreach loop instead of for loop in array's append! due to it being type-stable.

[LilithHafner]

Thanks, @prashantrahul141, and welcome! A lovely PR.

[adienes]

with this merged, I would suggest #51395 really needs to become a priority as now Base implicitly assumes that foreach operates in iteration order. which seems fine to me, but it should be codified / documented

[LilithHafner]

While I agree that we should handle #51395, it is okay for Base to depend on its own undocumented internals so this doesn't add much to #51395's urgency.