Modify task extensions to support backgroundTask

Modifies the extensions to TaskBuilder to extend TaskBuilderBase
instead, so they apply to both the task and backgroundTask builders.

Author: amongonz

src/FSharp.Control.TaskSeq/TaskExtensions.fs

@@ -14,8 +14,8 @@ open Microsoft.FSharp.Core.LanguagePrimitives.IntrinsicOperators
 [<AutoOpen>]
 module TaskExtensions =
 
-    // Add asynchronous for loop to the 'task' computation builder
-    type Microsoft.FSharp.Control.TaskBuilder with
+    // Add asynchronous for loop to the 'task' and 'backgroundTask' computation builders
+    type TaskBuilderBase with
 
         /// Used by `For`. F# currently doesn't support `while!`, so this cannot be called directly from the task CE
         /// This code is mostly a copy of TaskSeq.WhileAsync.

src/FSharp.Control.TaskSeq/TaskExtensions.fsi

@@ -5,7 +5,7 @@ namespace FSharp.Control
 [<AutoOpen>]
 module TaskExtensions =
 
-    type TaskBuilder with
+    type TaskBuilderBase with
 
         /// <summary>
         /// Inside <see cref="task" />, iterate over all values of a <see cref="taskSeq" />.