Show inline completions when completion menu is visible (#21858)

This changes the behavior of how we display inline completions and
non-inline completions (i.e. completion menu).

Previously we would never show inline completions if a completion menu
was visible, meaning that we'd never show Copilot/Supermaven/...
suggestions if the language server had a suggestion.

With this change, we now display the inline completions even if there is
a completion menu visible.

In that case `<tab>` then accepts the inline completion and `<enter>`
accepts the selected entry in the completion menu.

Release Notes:

- Changed how inline completions (Copilot, Supermaven, ...) and normal
completions (from language servers) interact. Zed will now also show
inline completions when the completion menu is visible. The user can
accept the inline completion with `<tab>` and the active entry in the
completion menu with `<enter>`. Previously, `<tab>` would also select
the active entry in the completion menu.

---------

Co-authored-by: Antonio <antonio@zed.dev>

assets/keymaps/default-linux.json

@@ -468,13 +468,21 @@
   },
   {
     "context": "Editor && showing_completions",
+    "use_key_equivalents": true,
+    "bindings": {
+      "enter": "editor::ConfirmCompletion"
+    }
+  },
+  {
+    "context": "Editor && !inline_completion && showing_completions",
+    "use_key_equivalents": true,
     "bindings": {
-      "enter": "editor::ConfirmCompletion",
       "tab": "editor::ComposeCompletion"
     }
   },
   {
-    "context": "Editor && inline_completion && !showing_completions",
+    "context": "Editor && inline_completion",
+    "use_key_equivalents": true,
     "bindings": {
       "tab": "editor::AcceptInlineCompletion"
     }