Extract ExtensionSlashCommand to assistant_slash_command crate (#20617)

This PR extracts the `ExtensionSlashCommand` implementation to the
`assistant_slash_command` crate.

The slash command related methods have been added to the `Extension`
trait. We also create separate data types for the slash command data
within the `extension` crate so that we can talk about them without
depending on the `extension_host` or `assistant_slash_command`.

Release Notes:

- N/A

Cargo.lock

@@ -459,8 +459,10 @@ name = "assistant_slash_command"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "async-trait",
  "collections",
  "derive_more",
+ "extension",
  "futures 0.3.30",
  "gpui",
  "language",
@@ -469,6 +471,7 @@ dependencies = [
  "pretty_assertions",
  "serde",
  "serde_json",
+ "ui",
  "workspace",
 ]