Introduce "Near" block type (#28032)

A "Near" block acts similarly to a "Below" block, but can (if it's
height is <= one line height) be shown on the end of the preceding line
instead of adding an entire blank line to the editor.

You can test it out by pasting this into `go_to_diagnostic_impl` and
then press `F8`
```
        let buffer = self.buffer.read(cx).snapshot(cx);
        let selection = self.selections.newest_anchor();

        self.display_map.update(cx, |display_map, cx| {
            display_map.insert_blocks(
                [BlockProperties {
                    placement: BlockPlacement::Near(selection.start),
                    height: Some(1),
                    style: BlockStyle::Flex,
                    render: Arc::new(|_| {
                        div()
                            .w(px(100.))
                            .h(px(16.))
                            .bg(gpui::hsla(0., 0., 1., 0.5))
                            .into_any_element()
                    }),
                    priority: 0,
                }],
                cx,
            )
        });
        return;
```

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/assistant_context_editor/src/context_editor.rs

@@ -1562,7 +1562,7 @@ impl ContextEditor {
                 })
             };
             let create_block_properties = |message: &Message| BlockProperties {
-                height: 2,
+                height: Some(2),
                 style: BlockStyle::Sticky,
                 placement: BlockPlacement::Above(
                     buffer
@@ -2111,7 +2111,7 @@ impl ContextEditor {
                     let image = render_image.clone();
                     anchor.is_valid(&buffer).then(|| BlockProperties {
                         placement: BlockPlacement::Above(anchor),
-                        height: MAX_HEIGHT_IN_LINES,
+                        height: Some(MAX_HEIGHT_IN_LINES),
                         style: BlockStyle::Sticky,
                         render: Arc::new(move |cx| {
                             let image_size = size_for_image(