Allow editor blocks to replace ranges of text (#19531)

This PR adds the ability for editor blocks to replace lines of text, but
does not yet use that feature anywhere. We'll update assistant patches
to use replace blocks on another branch:
https://github.com/zed-industries/zed/tree/assistant-patch-replace-blocks

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Richard Feldman <richard@zed.dev>
Co-authored-by: Marshall Bowers <marshall@zed.dev>
Co-authored-by: Nathan Sobo <nathan@zed.dev>

crates/editor/src/hunk_diff.rs

@@ -17,7 +17,7 @@ use workspace::Item;
 
 use crate::{
     editor_settings::CurrentLineHighlight, hunk_status, hunks_for_selections, ApplyDiffHunk,
-    BlockDisposition, BlockProperties, BlockStyle, CustomBlockId, DiffRowHighlight, DisplayRow,
+    BlockPlacement, BlockProperties, BlockStyle, CustomBlockId, DiffRowHighlight, DisplayRow,
     DisplaySnapshot, Editor, EditorElement, ExpandAllHunkDiffs, GoToHunk, GoToPrevHunk, RevertFile,
     RevertSelectedHunks, ToDisplayPoint, ToggleHunkDiff,
 };
@@ -417,10 +417,9 @@ impl Editor {
         };
 
         BlockProperties {
-            position: hunk.multi_buffer_range.start,
+            placement: BlockPlacement::Above(hunk.multi_buffer_range.start),
             height: 1,
             style: BlockStyle::Sticky,
-            disposition: BlockDisposition::Above,
             priority: 0,
             render: Box::new({
                 let editor = cx.view().clone();
@@ -700,10 +699,9 @@ impl Editor {
         let hunk = hunk.clone();
         let height = editor_height.max(deleted_text_height);
         BlockProperties {
-            position: hunk.multi_buffer_range.start,
+            placement: BlockPlacement::Above(hunk.multi_buffer_range.start),
             height,
             style: BlockStyle::Flex,
-            disposition: BlockDisposition::Above,
             priority: 0,
             render: Box::new(move |cx| {
                 let width = EditorElement::diff_hunk_strip_width(cx.line_height());