Add --task-border property (#837)

Author: beth-panx

packages/mgt-components/src/components/mgt-tasks-base/mgt-tasks-base.scss

@@ -150,6 +150,7 @@ $task-border-completed: var(--task-complete-border, 2px dotted inherit);
     margin: var(--task-margin, 0 0 18px 0);
     padding: var(--task-padding, 0 0 0 0);
     background-color: $background-color;
+    border: set-var(border, $theme-default, $tasks);
 
     .TaskContent {
       display: flex;

packages/mgt-components/src/components/mgt-tasks-base/mgt-tasks-base.theme.scss

@@ -14,6 +14,11 @@ $button__color--cancel: var(--task-new-cancel-button-color, set-var(color__sub1,
 $detail__color: var(--task-detail-color, set-var(color__sub2, $theme-default, $common));
 
 $tasks: (
+  border: (
+    _var: --task-border,
+    light: 2px solid $background-color__sub3--light,
+    dark: 2px solid $background-color__sub3--dark
+  ),
   border--completed: (
     _var: --task-complete-border,
     light: 2px dotted $background-color__sub3--light,