feature: support for text metrics

[hololeo]

Flet should support text metrics to allow for more precise layout of text.

Use case: creating a 'pager' of text content where user can displays one page at a time rather than scrolling. User can pick font size, and app can know where to cut off text blocks for each screen size based on metrics

Flutter links: https://api.flutter.dev/flutter/rendering/RenderParagraph-class.html https://medium.com/swlh/flutter-line-metrics-fd98ab180a64 https://www.google.com/search?q=flutter+text+metrics https://api.flutter.dev/flutter/dart-html/TextMetrics-class.html