Enable User Script Sandboxing

[dfeinzimer]

As of Xcode 15 User Script Sandboxing is enabled in new projects by default.

This can be set in Xcode via $PROJECT/$TARGET -> Build Settings -> Build Options -> User Script Sandboxing.

Presently it's disabled within all Toolkit projects & targets except for the test runner as there're access control issues stemming from the way we consume SwiftLint. We'd need to manage input/output files and file lists in the lint sources scripts to avoid build errors.

Once #191 is complete and SwiftLint is implemented as a package plugin we should be able to enable User Script Sandboxing without issue.