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/assistant/Cargo.toml

@@ -16,18 +16,19 @@ doctest = false
 anyhow.workspace = true
 anthropic = { workspace = true, features = ["schemars"] }
 assistant_slash_command.workspace = true
+async-watch.workspace = true
 cargo_toml.workspace = true
 chrono.workspace = true
 client.workspace = true
 collections.workspace = true
 command_palette_hooks.workspace = true
 editor.workspace = true
-feature_flags.workspace = true
 file_icons.workspace = true
 fs.workspace = true
 futures.workspace = true
 fuzzy.workspace = true
 gpui.workspace = true
+heed.workspace = true
 http.workspace = true
 indoc.workspace = true
 language.workspace = true
@@ -59,7 +60,6 @@ util.workspace = true
 uuid.workspace = true
 workspace.workspace = true
 picker.workspace = true
-gray_matter = "0.2.7"
 
 [dev-dependencies]
 ctor.workspace = true