Add intersperse method to Iterable extension for efficient element separation

Implement a new `intersperse` method that lazily inserts a specified separator between elements of an iterable. This method is memory-efficient and utilizes a generator for performance. Include comprehensive tests covering various scenarios, including empty iterables and different separator types.

Author: Abbas Hosseini

pkgs/collection/lib/src/iterable_extensions.dart

@@ -613,6 +613,33 @@ extension IterableExtension<T> on Iterable<T> {
       yield slice;
     }
   }
+
+  /// Lazily returns a new iterable where a [separator] is placed between each
+  /// element of this iterable.
+  ///
+  /// This is the most performant way to implement this functionality because
+  /// it uses a generator (`sync*`) and does not create a new list upfront,
+  /// making it memory-efficient for iterables of any size.
+  ///
+  /// Example:
+  /// ```dart
+  /// final numbers = [1, 2, 3];
+  /// final withSeparators = numbers.intersperse(0);
+  /// print(withSeparators.toList()); // Output: [1, 0, 2, 0, 3]
+  /// ```
+  ///
+  /// The resulting iterable is lazy. It only computes the next value when
+  /// it's requested.
+  Iterable<T> intersperse(T separator) sync* {
+    final iterator = this.iterator;
+    if (iterator.moveNext()) {
+      yield iterator.current;
+      while (iterator.moveNext()) {
+        yield separator;
+        yield iterator.current;
+      }
+    }
+  }
 }
 
 /// Extensions that apply to iterables with a nullable element type.

pkgs/collection/test/extensions_test.dart

@@ -1224,6 +1224,33 @@ void main() {
           );
         });
       });
+      group('.intersperse', () {
+        test('empty', () {
+          expect(iterable(<int>[]).intersperse(0), []);
+        });
+        test('single element', () {
+          expect(iterable([1]).intersperse(0), [1]);
+        });
+        test('two elements', () {
+          expect(iterable([1, 2]).intersperse(0), [1, 0, 2]);
+        });
+        test('multiple elements', () {
+          expect(iterable([1, 2, 3, 4]).intersperse(0), [1, 0, 2, 0, 3, 0, 4]);
+        });
+        test('with different separator types', () {
+          expect(iterable(['a', 'b', 'c']).intersperse('-'),
+              ['a', '-', 'b', '-', 'c']);
+          expect(iterable([1.5, 2.5, 3.5]).intersperse(0.0),
+              [1.5, 0.0, 2.5, 0.0, 3.5]);
+        });
+        test('with nullable int separator', () {
+          expect(iterable([1, 2, 3]).intersperse(0), [1, 0, 2, 0, 3]);
+        });
+        test('with empty string separator', () {
+          expect(iterable(['a', 'b', 'c']).intersperse(''),
+              ['a', '', 'b', '', 'c']);
+        });
+      });
     });
     group('of MapEntry', () {
       group('.whereKey', () {