Can I inject js code before the document loads?

`evalJavaScript` must be run after the page has finished loading. But i want to inject javascript code before the document loads. Reference:

```swift
WKUserScript(source: sourceJs, injectionTime: .atDocumentStart, forMainFrameOnly: false)
```

so do we provide this flutter api?

Start by tracing the existing evalJavaScript API and the native WebView integration; the issue points to WKUserScript with .atDocumentStart as the requested behavior. Check whether the plugin has a documented entry point or tests for JavaScript injection. Done means establishing whether Flutter can expose injection before document loading, with the behavior verified across the relevant platform.

Written by the indexing model from the issue text.