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/ui/src/components/keybinding.rs

@@ -77,7 +77,7 @@ impl RenderOnce for KeyBinding {
                         .join(" ")
                 )
             })
-            .gap(rems(0.125))
+            .gap(Spacing::Small.rems(cx))
             .flex_none()
             .children(self.key_binding.keystrokes().iter().map(|keystroke| {
                 let key_icon = Self::icon_for_key(keystroke);