Add a command to expand the context for a multibuffer (#10300)

This PR adds an action to expand the excerpts lines of context in a
multibuffer.

Release Notes:

- Added an `editor::ExpandExcerpts` action (bound to `shift-enter` by
default), which can expand the excerpt the cursor is currently in by 3
lines. You can customize the number of lines by rebinding this action
like so:

```json5
// In your keybindings array...
  {
    "context": "Editor && mode == full",
    "bindings": {
      "shift-enter": ["editor::ExpandExcerpts", { "lines": 5 }],
    }
  }
```

---------

Co-authored-by: Nathan <nathan@zed.dev>
Co-authored-by: Max <max@zed.dev>

crates/ui/src/components/icon.rs

@@ -47,6 +47,7 @@ pub enum IconName {
     ChevronLeft,
     ChevronRight,
     ChevronUp,
+    ExpandVertical,
     Close,
     Collab,
     Command,
@@ -149,6 +150,7 @@ impl IconName {
             IconName::ChevronLeft => "icons/chevron_left.svg",
             IconName::ChevronRight => "icons/chevron_right.svg",
             IconName::ChevronUp => "icons/chevron_up.svg",
+            IconName::ExpandVertical => "icons/expand_vertical.svg",
             IconName::Close => "icons/x.svg",
             IconName::Collab => "icons/user_group_16.svg",
             IconName::Command => "icons/command.svg",