[SuperEditor][SuperReader] - Implement line-based layout

[matthew-carroll]

Flutter implements block-based layout, even for test. In Flutter, one lays out an entire paragraph (or block) of text at one time.

Many features in rich text layout depend upon the flow of lines of text:

• Styled text runs within text runs
  • Inline code
  • Tags (users, docs, tasks)
• Inline bitmaps (think emojis but bitmaps)
• Text-embedded UI (injecting a button in the middle of text)
• A single block of text flowing around obstacles (like flowing around an image)

Flutter doesn't support these use-cases, except perhaps inline bitmaps. Flutter technically supports inline widgets but this support is almost useless when it comes to the needs of rich text layout and editing. Instead, what's needed is a completely different layout approach.

Ideally, Flutter would expose the full surface area of SkParagraph, which is the underlying text shaping library that Flutter uses. With full access to SkParagraph, we could probably build our own render object that calculates layout in a relatively efficient manner.

However, Flutter has only included a subset of SkParagraph. As a result, implementing line-based layout requires repeatedly measuring blocks of text, querying line breaks, injecting non-text content, and then continuing the process.

For this ticket, implement the aforementioned line-based layout by writing one or more custom render objects, which both layout content based on lines, and also reports any relevant bounds to other widgets.