Add slash commands for adding context into the assistant (#12102)

Tasks

* [x] remove old flaps and output when editing a slash command
* [x] the completing a command name that takes args, insert a space to
prepare for typing an arg
* [x] always trigger completions when  typing in a slash command
* [x] don't show line numbers
* [x] implement `prompt` command
* [x] `current-file` command
* [x] state gets corrupted on `duplicate line up` on a slash command
* [x] exclude slash command source from completion request

Next steps:
* show output token count in flap trailer
* add `/project` command that matches project ambient context
* delete ambient context

Release Notes:

- N/A

---------

Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/assistant/src/prompt_library.rs

@@ -156,10 +156,10 @@ impl PromptLibrary {
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq)]
 pub struct UserPrompt {
     version: String,
-    title: String,
+    pub title: String,
     author: String,
     languages: Vec<String>,
-    prompt: String,
+    pub prompt: String,
 }
 
 impl UserPrompt {