Add flex_grow and flex_shrink.

Two new elements:
- flex_grow  : Expand the element to occupy free space.
- flex_shrink: Minimize the element leave away missing space.

flex = flex_grow | flex_shrink.

Other changes:
- hbox and vbox are now non flexible by default.
- the vtext element has been added to help writting tests.
- Many new tests.

src/ftxui/dom/size.cpp

@@ -37,10 +37,13 @@ class Size : public Node {
         break;
     }
 
-    if (direction_ == WIDTH)
-      requirement_.flex_x = 0;
-    else
-      requirement_.flex_y = 0;
+    if (direction_ == WIDTH) {
+      requirement_.flex_grow_x = 0;
+      requirement_.flex_shrink_x = 0;
+    } else {
+      requirement_.flex_grow_y = 0;
+      requirement_.flex_shrink_y = 0;
+    }
   }
 
   void SetBox(Box box) override {