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>

crates/editor/src/code_context_menus.rs

@@ -169,7 +169,6 @@ pub struct CompletionsMenu {
     resolve_completions: bool,
     show_completion_documentation: bool,
     last_rendered_range: Rc<RefCell<Option<Range<usize>>>>,
-    pub previewing_inline_completion: bool,
 }
 
 impl CompletionsMenu {
@@ -200,7 +199,6 @@ impl CompletionsMenu {
             scroll_handle: UniformListScrollHandle::new(),
             resolve_completions: true,
             last_rendered_range: RefCell::new(None).into(),
-            previewing_inline_completion: false,
         }
     }
 
@@ -257,7 +255,6 @@ impl CompletionsMenu {
             resolve_completions: false,
             show_completion_documentation: false,
             last_rendered_range: RefCell::new(None).into(),
-            previewing_inline_completion: false,
         }
     }
 
@@ -410,12 +407,8 @@ impl CompletionsMenu {
         .detach();
     }
 
-    pub fn is_empty(&self) -> bool {
-        self.entries.borrow().is_empty()
-    }
-
     pub fn visible(&self) -> bool {
-        !self.is_empty() && !self.previewing_inline_completion
+        !self.entries.borrow().is_empty()
     }
 
     fn origin(&self) -> ContextMenuOrigin {
@@ -709,10 +702,6 @@ impl CompletionsMenu {
         // This keeps the display consistent when y_flipped.
         self.scroll_handle.scroll_to_item(0, ScrollStrategy::Top);
     }
-
-    pub fn set_previewing_inline_completion(&mut self, value: bool) {
-        self.previewing_inline_completion = value;
-    }
 }
 
 #[derive(Clone)]