[SuperEditor][SuperReader] - Added support for inline widgets (Resolves #2442)

[matthew-carroll]

[SuperEditor][SuperReader] - Added support for inline widgets (Resolves #2442)

Screen.Recording.2024-12-09.at.3.09.48.PM.mov

How to customize inline widgets:
Similar to the inline text styler, the inline widget builders can be configured on the stylesheet.

final myStylesheet = defaultStylesheet.copyWith(
  inlineWidgetBuilderChain: [
    myBuilder1,
    myBuilder2,
    // etc.
  ],
);

Default inline widget builders:
Currently there are only 2 default inline widget builders. One for network images, and one for asset images.

Changes to uses of AttributedText:
Due to changes in AttributedText to support placeholders, this PR makes the following changes:

• myText.text.length -> myText.length (because the length of the String no longer matches the length of the AttributedText)
• myText.text -> myText.toPlainText() (because there's no longer a single possible plain-text representation)

[angelosilvestre]

LGTM