T21b and T21c: Documents, reading and knowledge

- DocumentViewer: pages the app drew on a desk, the page under the view's middle
  counted, zoom steps and fit, thumbnails, notes pinned where a press lands, and
  the owner's FindWidget over the pages, hits lit and the current one revealed
- PageThumbnailList, EpubReader (chapters, type size, progress), PrintPreview
- MarkdownRenderer feeds TableOfContents through sections and outline
- Footnote, TooltipTerm, Glossary, ReadingProgress, ZenMode
- Backlinks, Favorites, TrashBin, PageCover, PageIcon, TemplatePicker,
  VersionHistory with PageHistoryDiff
- editor::FindWidget shows the replace fold and option toggles only with handlers
- block editor tests move to documents/blocks
- icons: a-arrow-down, a-arrow-up
- gallery: reading and knowledge sections; scripted find, changes and zen

src/documents/blocks/mod.rs

@@ -4,6 +4,8 @@ mod keys;
 mod kind;
 mod media;
 mod table;
+#[cfg(all(test, feature = "test-support"))]
+mod tests;
 mod view;
 
 use std::{collections::HashMap, rc::Rc};
@@ -15,6 +17,7 @@ use gpui::{
 
 pub use kind::{Align, BlockKind, Media};
 pub(crate) use kind::{matching, shortcut};
+pub(crate) use media::source;
 
 use super::markdown::markdown_highlights;
 use crate::forms::{History, InputEvent, TextInput, code_highlights};