feat(Dictionary): add storing:by: init

Sources/HelperKit/Extensions/Dictionary.swift

@@ -28,4 +28,34 @@ extension Dictionary {
 
         self = result
     }
+
+    /// Creates a new dictionary whose keys are the properties returned by the
+    /// given closure and whose values are the elements that returned each key.
+    ///
+    /// If two elements return the same key, only one will be kept. The closure
+    /// should return a unique enough key, like an ID.
+    ///
+    ///     struct Book: Identifiable {
+    ///         let id: Int
+    ///         let name: String
+    ///     }
+    ///
+    ///     let books = [Book(id: 1, name: "One"), Book(id: 2, name: "Two"), Book(id: 3, name: "Three")]
+    ///     let dict1 = Dictionary(storing: books, by: \.id)
+    ///     // [1: Book(id: 1, name: "One"), 2: Book(id: 2, name: "Two")]
+    ///     let dict2 = Dictionary(storing: books, by: { $0.name.count * 2 })
+    ///     // [6: Book(id: 2, name: "Two"), 10: Book(id: 3, name: "Three")]
+    ///
+    /// - Parameters:
+    ///   - values: A sequence of values to store in a dictionary.
+    ///   - keyForValue: A closure that returns a key for each element in `values`.
+    @inlinable public init<S>(storing values: S, by keyForValue: (S.Element) -> Key) where S: Sequence, S.Element == Value {
+        var result: [Key: Value] = [:]
+
+        for value in values {
+            result[keyForValue(value)] = value
+        }
+
+        self = result
+    }
 }