[Segmented] Proportional SegmentItem sizing

### Describe the bug

Allow developers to easily adjust the sizing of segmented items relative to one another.

### Steps to reproduce

```text
N/A
```

### Expected behavior

Certain segmented items would be proportionally larger than others.

### Screenshots

### Code Platform

- [x] UWP
- [x] WinAppSDK / WinUI 3
- [ ] Web Assembly (WASM)
- [ ] Android
- [ ] iOS
- [ ] MacOS
- [ ] Linux / GTK

### Windows Build Number

- [ ] Windows 10 1809 (Build 17763)
- [ ] Windows 10 1903 (Build 18362)
- [ ] Windows 10 1909 (Build 18363)
- [ ] Windows 10 2004 (Build 19041)
- [ ] Windows 10 20H2 (Build 19042)
- [ ] Windows 10 21H1 (Build 19043)
- [ ] Windows 10 21H2 (Build 19044)
- [ ] Windows 10 22H2 (Build 19045)
- [ ] Windows 11 21H2 (Build 22000)
- [ ] Other (specify)

### Other Windows Build number

_No response_

### App minimum and target SDK version

- [ ] Windows 10, version 1809 (Build 17763)
- [ ] Windows 10, version 1903 (Build 18362)
- [ ] Windows 10, version 1909 (Build 18363)
- [ ] Windows 10, version 2004 (Build 19041)
- [ ] Windows 10, version 2104 (Build 20348)
- [ ] Windows 11, version 22H2 (Build 22000)
- [ ] Other (specify)

### Other SDK version

_No response_

### Visual Studio Version

_No response_

### Visual Studio Build Number

_No response_

### Device form factor

_No response_

### Additional context

_No response_

### Help us help you

Yes, I'd like to be assigned to work on this item.

Issue では Segmented と SegmentItem の名前が挙げられていますが、ファイルやテストは特定されていません。まずこれらのコントロールの実装を見つけ、項目のサイズが現在どのように計算されているかを確認し、UWP と WinUI 3 で Segmented の項目を異なる比例サイズでレンダリングできることを完了条件として定義してください。

索引モデルが issue の本文から書いたものです。