Error early when using conflicting architectures

[kraenhansen]

As a temporary workaround to fail faster and provide a better suggested fix in case a user builds both the arm64 and x86_64 slices for a simulator, I suggest adding an assertion to check for conflicting architectures before configuring the projects.

After merging this PR, running

npx cmake-rn --triplet arm64-apple-ios-sim --triplet x86_64-apple-ios-sim

will yield

✖ Failed to configure projects: Conflicting triplet variants for apple-ios-sim

ERROR Conflicting triplet variants for apple-ios-sim
FIX Remove either the arm64 or x86_64 variant of the apple-ios-sim triplet or use the universal 'arm64;x86_64-apple-ios-sim' triplet

and

npx cmake-rn --triplet arm64-apple-ios-sim --triplet 'arm64;x86_64-apple-ios-sim'

will yield

✖ Failed to configure projects: Conflicting triplet variants for apple-ios-sim

ERROR Conflicting triplet variants for apple-ios-sim
FIX Remove the arm64 variant of the apple-ios-sim triplet and use the universal 'arm64;x86_64-apple-ios-sim' triplet instead