Add .swiftpm/config/registries.json to default .gitignore (#3468)

As discussed in SE-0292, a user may inadvertently reveal the existence of a private registry or expose hardcoded credentials by checking in their project's .swiftpm/config/registries.json file or directory. By adding this entry to the .gitignore file in the new project template, we significantly minimize the possibility of the user leaking credentials in the future.

Author: mattt

Sources/Workspace/InitPackage.swift

@@ -260,6 +260,7 @@ public final class InitPackage {
                 /*.xcodeproj
                 xcuserdata/
                 DerivedData/
+                .swiftpm/config/registries.json
                 .swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata
                 .netrc