A set of outline panel fixes (#12965)

Follow-up of https://github.com/zed-industries/zed/pull/12637

* Wrong font size for the outline items (fixes
https://github.com/zed-industries/zed/pull/12637#issuecomment-2164084021)
* Duplicate context menu item (fixes
https://github.com/zed-industries/zed/issues/12957)
* Missing `space` keybinding for item opening (fixes
https://github.com/zed-industries/zed/issues/12956)
* Adds 60px more to the default width (fixes
https://github.com/zed-industries/zed/issues/12955)
* Incorrect scroll for singleton buffers (fixes
https://github.com/zed-industries/zed/issues/12953)

Release Notes:

- N/A

assets/keymaps/default-linux.json

@@ -566,11 +566,12 @@
   {
     "context": "OutlinePanel",
     "bindings": {
-      "left": "project_panel::CollapseSelectedEntry",
-      "right": "project_panel::ExpandSelectedEntry",
-      "ctrl-alt-c": "project_panel::CopyPath",
-      "alt-ctrl-shift-c": "project_panel::CopyRelativePath",
-      "alt-ctrl-r": "project_panel::RevealInFinder",
+      "left": "outline_panel::CollapseSelectedEntry",
+      "right": "outline_panel::ExpandSelectedEntry",
+      "ctrl-alt-c": "outline_panel::CopyPath",
+      "alt-ctrl-shift-c": "outline_panel::CopyRelativePath",
+      "alt-ctrl-r": "outline_panel::RevealInFinder",
+      "space": "outline_panel::Open",
       "shift-down": "menu::SelectNext",
       "shift-up": "menu::SelectPrev"
     }