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>

Cargo.toml

@@ -150,6 +150,7 @@ assets = { path = "crates/assets" }
 assistant = { path = "crates/assistant" }
 assistant_slash_command = { path = "crates/assistant_slash_command" }
 assistant_tooling = { path = "crates/assistant_tooling" }
+async-watch = "0.3.1"
 audio = { path = "crates/audio" }
 auto_update = { path = "crates/auto_update" }
 base64 = "0.13"
@@ -166,6 +167,7 @@ color = { path = "crates/color" }
 command_palette = { path = "crates/command_palette" }
 command_palette_hooks = { path = "crates/command_palette_hooks" }
 copilot = { path = "crates/copilot" }
+dashmap = "5.5.3"
 db = { path = "crates/db" }
 diagnostics = { path = "crates/diagnostics" }
 editor = { path = "crates/editor" }