ios build: Move "Start Metro" script into a script file, make it not run for release build.

[chrisbobbe]

In 33f4b41, we added a build phase to our own project to start Metro, since this had previously been done by a build phase in React.xcodeproj, and we stopped using React.xcodeproj and all other .xcodeprojs except ZulipMobile and Pods, since we were moving over to CocoaPods. The new script was added correctly, according to React Native, by following their template, but Greg points out some improvements we can still make, then make a nice PR upstream:

Greg said:

I took a look at [the one in React.xcodeproj] for comparison. It has some helpful features this version lacks, which would be nice to incorporate. (Notably, it avoids bothering to launch the packager for a release build.) ... ha, apparently I was looking at a v0.60 version of React.xcodeproj, oops 🙂 Anyway, that is a good feature.

This is an unfriendly spot to have to edit it, though. It'd be much cleaner as a separate script file.

Anyway, doing anything about that can be a followup. (And then a good PR upstream, I think!)

[chrisbobbe]

I've posted at react-native-community/cli#317 (comment) with the hope that React Native will reconsider a once-considered idea to use a CocoaPods pattern that would mean we don't have to do anything here (except make a nice PR to help along the process, if they ask us to 🙂).

facebook/react-native@4ea6204 was announced in the iOS-specific section of the RN v0.60 release notes, as "Do not run packager in Release mode". But, unless I've quite misunderstood, nobody actually got that benefit without a fair amount of investigation and manual tweaks that aren't mentioned at all in the upgrade guide.