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/Cargo.toml

@@ -24,9 +24,9 @@ test-support = [
 anyhow.workspace = true
 assistant_settings.workspace = true
 assistant_slash_command.workspace = true
+assistant_slash_commands.workspace = true
 assistant_tool.workspace = true
 async-watch.workspace = true
-cargo_toml.workspace = true
 chrono.workspace = true
 client.workspace = true
 clock.workspace = true
@@ -39,10 +39,7 @@ 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
 indoc.workspace = true
 language.workspace = true
@@ -51,7 +48,6 @@ language_model_selector.workspace = true
 language_models.workspace = true
 log.workspace = true
 lsp.workspace = true
-markdown.workspace = true
 menu.workspace = true
 multi_buffer.workspace = true
 open_ai = { workspace = true, features = ["schemars"] }
@@ -82,7 +78,6 @@ terminal.workspace = true
 terminal_view.workspace = true
 text.workspace = true
 theme.workspace = true
-toml.workspace = true
 ui.workspace = true
 util.workspace = true
 uuid.workspace = true