Fix iOS linking errors by bumping minimum Swift version to 5.0 (#23)

* Add `Package@swift-5.5.swift` manifest

Package@swift-5.5.swift

@@ -0,0 +1,15 @@
+// swift-tools-version:5.0
+import PackageDescription
+
+let name = "LegibleError"
+
+let package = Package(
+    name: name,
+    products: [
+        .library(name: name, targets: [name]),
+    ],
+    targets: [
+        .target(name: name, path: "Sources"),
+        .testTarget(name: "LegibleErrorTests", dependencies: [.init(stringLiteral: name)]),
+    ]
+)