Allow flattening CollectionNodes

Author: Simon Anreiter

Sources/SwiftSyntaxBuilder/BuildableCollectionNodes.swift.gyb

@@ -39,6 +39,13 @@ public struct ${type.buildable()}: ExpressibleByArrayLiteral, SyntaxBuildable, $
     self.elements = elements.map { $0.create${element_type.buildable_base_name()}() }
 %   end
   }
+  
+  /// Creates a new ${type.buildable()} by flattening the elements in `lists`
+  public init(combining lists: [${type.expressible_as()}]) {
+    self.elements = lists.flatMap {
+      $0.create${type.buildable()}().elements
+    }
+  }
 
   public init(arrayLiteral elements: ${element_type.expressible_as()}...) {
     self.init(elements)

Sources/SwiftSyntaxBuilder/ResultBuilders.swift.gyb

@@ -49,7 +49,12 @@ public struct ${type.result_builder()} {
   public static func buildExpression(_ expression: Expression) -> Component {
     return [expression]
   }
-
+  
+  /// Add all the elements of `expression` to this result builder, effectively flattening them.
+  public static func buildExpression(_ expression: ${type.expressible_as()}) -> Component {
+    return expression.create${type.buildable()}().elements
+  }
+  
   /// Enables support for `if` statements that do not have an `else`.
   public static func buildOptional(_ component: Component?) -> Component {
     return component ?? []