[webview_flutter_web] Add WebWebViewController.runJavaScript

[sophiebremer]

This PR (replaces #9835) adds an implementation for WebWebViewController.runJavaScript and changes the way WebWebViewController.loadHtmlString injects HTML code while also adding support for the baseUrl option.

The following issues are fixed:

• https://github.com/flutter/flutter/issues/173899

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[sophiebremer]

Hello @mdebbar!

Please provide guidance. Not all tests are successful, and how to add a useful test for WebWebViewController.runJavaScript without browser context is unclear to me.