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>

assets/keymaps/default-linux.json

@@ -216,6 +216,13 @@
       "alt-enter": "editor::Newline"
     }
   },
+  {
+    "context": "PromptLibrary",
+    "bindings": {
+      "ctrl-n": "prompt_library::NewPrompt",
+      "ctrl-shift-s": "prompt_library::ToggleDefaultPrompt"
+    }
+  },
   {
     "context": "BufferSearchBar",
     "bindings": {