Extract slash commands to their own crate (#23261)

This PR extracts the slash command definitions out of the `assistant`
crate and into their own `assistant_slash_commands` crate.

Release Notes:

- N/A

crates/assistant_slash_commands/Cargo.toml

@@ -0,0 +1,52 @@
+[package]
+name = "assistant_slash_commands"
+version = "0.1.0"
+edition = "2021"
+publish = false
+license = "GPL-3.0-or-later"
+
+[lints]
+workspace = true
+
+[lib]
+path = "src/assistant_slash_commands.rs"
+
+[dependencies]
+anyhow.workspace = true
+assistant_slash_command.workspace = true
+cargo_toml.workspace = true
+chrono.workspace = true
+collections.workspace = true
+context_server.workspace = true
+editor.workspace = true
+feature_flags.workspace = true
+fs.workspace = true
+futures.workspace = true
+fuzzy.workspace = true
+globset.workspace = true
+gpui.workspace = true
+html_to_markdown.workspace = true
+http_client.workspace = true
+indexed_docs.workspace = true
+language.workspace = true
+language_model.workspace = true
+log.workspace = true
+project.workspace = true
+prompt_library.workspace = true
+rope.workspace = true
+schemars.workspace = true
+semantic_index.workspace = true
+serde.workspace = true
+serde_json.workspace = true
+smol.workspace = true
+terminal_view.workspace = true
+text.workspace = true
+toml.workspace = true
+ui.workspace = true
+util.workspace = true
+workspace.workspace = true
+
+[dev-dependencies]
+env_logger.workspace = true
+pretty_assertions.workspace = true
+settings.workspace = true