Make slash command descriptions consistent (#18595)

This PR adds a description constant in most of the slash command files
so that both the editor _and_ footer pickers use the same string. In
terms of copywriting, I did some tweaking to reduce the longer ones a
bit. Also standardized them all to use sentence case, as opposed to each
instance using a different convention. The editor picker needs more
work, though, given the arguments and descriptions are being cut at the
moment. This should happen in a follow-up!

<img width="900" alt="Screenshot 2024-10-01 at 7 25 19 PM"
src="https://github.com/user-attachments/assets/e8759eff-0de9-4a4d-a026-366d85507b3c">

---

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/assistant/src/slash_command/file_command.rs

@@ -110,11 +110,11 @@ impl SlashCommand for FileSlashCommand {
     }
 
     fn description(&self) -> String {
-        "insert file".into()
+        "Insert file".into()
     }
 
     fn menu_text(&self) -> String {
-        "Insert File".into()
+        self.description()
     }
 
     fn requires_argument(&self) -> bool {