assistant: Put /docs and /project behind a setting (#16186)

This PR puts the availability of the `/docs` and `/project` slash
commands behind their respective settings.

Release Notes:

- N/A

crates/assistant/src/slash_command_settings.rs

@@ -0,0 +1,44 @@
+use anyhow::Result;
+use gpui::AppContext;
+use schemars::JsonSchema;
+use serde::{Deserialize, Serialize};
+use settings::{Settings, SettingsSources};
+
+/// Settings for slash commands.
+#[derive(Deserialize, Serialize, Debug, Default, Clone, JsonSchema)]
+pub struct SlashCommandSettings {
+    /// Settings for the `/docs` slash command.
+    #[serde(default)]
+    pub docs: DocsCommandSettings,
+    /// Settings for the `/project` slash command.
+    #[serde(default)]
+    pub project: ProjectCommandSettings,
+}
+
+/// Settings for the `/docs` slash command.
+#[derive(Deserialize, Serialize, Debug, Default, Clone, JsonSchema)]
+pub struct DocsCommandSettings {
+    /// Whether `/docs` is enabled.
+    #[serde(default)]
+    pub enabled: bool,
+}
+
+/// Settings for the `/project` slash command.
+#[derive(Deserialize, Serialize, Debug, Default, Clone, JsonSchema)]
+pub struct ProjectCommandSettings {
+    /// Whether `/project` is enabled.
+    #[serde(default)]
+    pub enabled: bool,
+}
+
+impl Settings for SlashCommandSettings {
+    const KEY: Option<&'static str> = Some("slash_commands");
+
+    type FileContent = Self;
+
+    fn load(sources: SettingsSources<Self::FileContent>, _cx: &mut AppContext) -> Result<Self> {
+        SettingsSources::<Self::FileContent>::json_merge_with(
+            [sources.default].into_iter().chain(sources.user),
+        )
+    }
+}