Add generated xcshareddata folder to gitignore (facebook#25451)

Summary:
A new project created with `0.60.0-rc.3` generates an `xcshareddata` when you open the workspace in Xcode at `ios/ProjectName.xcworkspace/xcshareddata`. This does not need to be tracked in git.

## Changelog

[IOS] [ADDED] - Add generated `xcshareddata` folder to gitignore
Pull Request resolved: facebook#25451

Test Plan: N/A

Differential Revision: D16110125

Pulled By: hramos

fbshipit-source-id: 52ac4d2d7734956817f875c3c7bbf341cba2ea28

template/_gitignore

@@ -20,7 +20,7 @@ DerivedData
 *.hmap
 *.ipa
 *.xcuserstate
-project.xcworkspace
+xcshareddata
 
 # Android/IntelliJ
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