feat: ion-list on ios should have top and bottom borders

还没有人认领这个 Issue。

# Bug Report

**Ionic version:**

[ ] **4.x**
[x] **5.x**

**Current behavior:**

In native IOS apps there are two full lines on top and bottom of a list. In Ionic the top line is completely missing and bottom line is inset.

### Native IOS list

### Ionic list

**Expected behavior:**

Without providing any attributes to ion-list and ion-item, the default list should look like in first screenshot:
- line on top and bottom of a list is full
- item lines are inset

**Steps to reproduce:**

**Related code:**

Just copy paste this snippet to latest Ionic version.

```



example@mail.net

```

***My current workaround***

### .html

```

```

### .css

```
.ios-list {
border: solid;
border-color: var(
--ion-item-border-color,
var(--ion-border-color, var(--ion-color-step-250, #c8c7cc))
);
border-width: 0.55px 0px 0.55px 0px;
}
```
**Ionic info:**

```
Ionic:

Ionic CLI : 6.10.0 (/usr/local/lib/node_modules/@ionic/cli)
Ionic Framework : @ionic/angular 5.1.1
@angular-devkit/build-angular : 0.901.7
@angular-devkit/schematics : 8.3.26
@angular/cli : 9.1.7
@ionic/angular-toolkit : 2.2.0

Capacitor:

Capacitor CLI : 2.0.1
@capacitor/core : 2.0.1

Utility:

cordova-res (update available: 0.15.1) : 0.14.0
native-run : not installed

System:

NodeJS : v12.16.0 (/usr/local/bin/node)
npm : 6.14.5
OS : macOS Catalina
```

从 iOS 模式的 Ionic 5 应用中提供的 ion-list 和 ion-item 代码片段开始，并将其顶部和底部边框与截图进行比较。完成的标准是：默认列表的顶部和底部边框均为全宽，同时 item 分隔线保持内缩，且不使用报告中的 workaround。

由索引模型根据 Issue 内容生成。