Start on a database-backed prompt library (#12468)

Using the file system as a database seems like it's easy, but it's
actually a real pain. I'd like to use LMDB to store the prompts locally
so we have more control. We can always add an export option, but I want
the source of truth to be somewhere other than the file system.

So far, I have a PromptStore which is global to the application and can
be initialized on startup. Then there's a `PromptLibrary` which is
intended to be the root of a new kind of Zed window. I haven't actually
seen pixels yet, but I've sketched out the basics needed to create a new
prompt, save, etc.

Still lots to figure out but the foundations of being backed by a DB and
rendering in an independent window are in place.

/cc @iamnbutler @as-cii 

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/editor/src/editor.rs

@@ -68,10 +68,10 @@ use gpui::{
     div, impl_actions, point, prelude::*, px, relative, size, uniform_list, Action, AnyElement,
     AppContext, AsyncWindowContext, AvailableSpace, BackgroundExecutor, Bounds, ClipboardItem,
     Context, DispatchPhase, ElementId, EventEmitter, FocusHandle, FocusableView, FontId, FontStyle,
-    FontWeight, HighlightStyle, Hsla, InteractiveText, KeyContext, Model, MouseButton, PaintQuad,
-    ParentElement, Pixels, Render, SharedString, Size, StrikethroughStyle, Styled, StyledText,
-    Subscription, Task, TextStyle, UnderlineStyle, UniformListScrollHandle, View, ViewContext,
-    ViewInputHandler, VisualContext, WeakView, WhiteSpace, WindowContext,
+    FontWeight, HighlightStyle, Hsla, InteractiveText, KeyContext, ListSizingBehavior, Model,
+    MouseButton, PaintQuad, ParentElement, Pixels, Render, SharedString, Size, StrikethroughStyle,
+    Styled, StyledText, Subscription, Task, TextStyle, UnderlineStyle, UniformListScrollHandle,
+    View, ViewContext, ViewInputHandler, VisualContext, WeakView, WhiteSpace, WindowContext,
 };
 use highlight_matching_bracket::refresh_matching_bracket_highlights;
 use hover_popover::{hide_hover, HoverState};
@@ -1113,7 +1113,8 @@ impl CompletionsMenu {
         .occlude()
         .max_h(max_height)
         .track_scroll(self.scroll_handle.clone())
-        .with_width_from_item(widest_completion_ix);
+        .with_width_from_item(widest_completion_ix)
+        .with_sizing_behavior(ListSizingBehavior::Infer);
 
         Popover::new()
             .child(list)
@@ -1460,6 +1461,7 @@ impl CodeActionsMenu {
                 })
                 .map(|(ix, _)| ix),
         )
+        .with_sizing_behavior(ListSizingBehavior::Infer)
         .into_any_element();
 
         let cursor_position = if let Some(row) = self.deployed_from_indicator {