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>

assets/icons/LICENSES

@@ -0,0 +1,9 @@
+Lucide License
+
+ISC License
+
+Copyright (c) for portions of Lucide are held by Cole Bemis 2013-2022 as part of Feather (MIT). All other copyright (c) for Lucide are held by Lucide Contributors 2022.
+
+Permission to use, copy, modify, and/or distribute this software for any purpose with or without fee is hereby granted, provided that the above copyright notice and this permission notice appear in all copies.
+
+THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

assets/icons/expand_vertical.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-unfold-vertical"><path d="M12 22v-6"/><path d="M12 8V2"/><path d="M4 12H2"/><path d="M10 12H8"/><path d="M16 12h-2"/><path d="M22 12h-2"/><path d="m15 19-3 3-3-3"/><path d="m15 5-3-3-3 3"/></svg>

assets/keymaps/default-linux.json

@@ -527,6 +527,7 @@
     "context": "Editor && mode == full",
     "bindings": {
       "alt-enter": "editor::OpenExcerpts",
+      "shift-enter": "editor::ExpandExcerpts",
       "ctrl-k enter": "editor::OpenExcerptsSplit",
       "ctrl-f8": "editor::GoToHunk",
       "ctrl-shift-f8": "editor::GoToPrevHunk",

assets/keymaps/default-macos.json

@@ -541,6 +541,7 @@
     "context": "Editor && mode == full",
     "bindings": {
       "alt-enter": "editor::OpenExcerpts",
+      "shift-enter": "editor::ExpandExcerpts",
       "cmd-k enter": "editor::OpenExcerptsSplit",
       "cmd-f8": "editor::GoToHunk",
       "cmd-shift-f8": "editor::GoToPrevHunk",