gpui: Fix `text-align` with nowrap mode (#24116)

Release Notes:

- N/A


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- Continue #24090 to fix text align for when used `whitespace_nowrap`.
- Fix wrapped line length calculation.

And add example

```
cargo run -p gpui --example text_layout
```

<img width="760" alt="image"
src="https://github.com/user-attachments/assets/a087c300-0e0e-4a80-98c6-90161a9b0905"
/>

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Co-authored-by: Owen Law <owenlaw222@gmail.com>