message: style(NavBar): improve text size adjust
diff: .van-nav-bar {
  position: relative;
  z-index: @nav-bar-z-index;
  + display: flex;
  + align-items: center;
  height: @nav-bar-height;
  - line-height: @nav-bar-height;
  + line-height: 1.5;
  text-align: center;
  background-color: @nav-bar-background-color;
  user-select: none;
  