[SuperEditor][SuperReader][SuperTextField] - Inline image and widget support

[matthew-carroll]

This is a meta ticket for the goal of inserting, selecting, and deleting small images and widgets within text in SuperEditor, SuperReader, and SuperTextField.

Small images and widgets means images and widgets whose size is comparable to the text around them.

Anti-goal: This work does NOT include text within widgets within text, e.g., inline code snippets. This work does NOT include document-level line flowing, e.g., paragraphs flowing around images.

Steps:

1. Add the concept of single character placeholders to AttributedText: [AttributedText] - Add single-character placeholders #2440
2. Adjust SuperText to produce InlineSpans that now include WidgetSpans, which inject the appropriate widgets for the placeholders in AttributedText: [SuperText] - Render inline widgets #2441
3. SuperEditor: Adjust selection rendering, content insertion, and content deletion behaviors to work with images: [SuperEditor][SuperReader] - Add support for inline images and gifs #2442
4. SuperTextField: Adjust selection rendering, content insertion, and content deletion behaviors to work with images: [SuperTextField] - Bring inline widgets to SuperTextField #2507