edit predictions: Preview while holding modifier mode (#24316)

This PR adds a new `inline_completions.inline_preview` config which can
be set to `auto` (current behavior) or to `when_holding_modifier`.
When set to the latter, instead of showing edit prediction previews
inline in the buffer, we'll show it in a popover (even when there's no
LSP completion) so your isn't constantly moving as completions arrive.


https://github.com/user-attachments/assets/3615d151-3633-4ee4-98b9-66ee0aa735b8

Release Notes:

- N/A

---------

Co-authored-by: Danilo <danilo@zed.dev>

assets/settings/default.json

@@ -783,7 +783,14 @@
       "**/*.cert",
       "**/*.crt",
       "**/secrets.yml"
-    ]
+    ],
+    // When to show edit predictions previews in buffer.
+    // This setting takes two possible values:
+    // 1. Display inline when there are no language server completions available.
+    //     "inline_preview": "auto"
+    // 2. Display inline when holding modifier key (alt by default).
+    //     "inline_preview": "when_holding_modifier"
+    "inline_preview": "auto"
   },
   // Settings specific to journaling
   "journal": {