Rename extension crate to extension_host (#20081)

This PR renames the `extension` crate to `extension_host`.

This is to free up the name so that we can create a smaller-scoped
`extension` crate.

Release Notes:

- N/A

crates/extension_host/src/extension_settings.rs

@@ -0,0 +1,51 @@
+use anyhow::Result;
+use collections::HashMap;
+use gpui::AppContext;
+use schemars::JsonSchema;
+use serde::{Deserialize, Serialize};
+use settings::{Settings, SettingsSources};
+use std::sync::Arc;
+
+#[derive(Deserialize, Serialize, Debug, Default, Clone, JsonSchema)]
+pub struct ExtensionSettings {
+    /// The extensions that should be automatically installed by Zed.
+    ///
+    /// This is used to make functionality provided by extensions (e.g., language support)
+    /// available out-of-the-box.
+    #[serde(default)]
+    pub auto_install_extensions: HashMap<Arc<str>, bool>,
+    #[serde(default)]
+    pub auto_update_extensions: HashMap<Arc<str>, bool>,
+}
+
+impl ExtensionSettings {
+    /// Returns whether the given extension should be auto-installed.
+    pub fn should_auto_install(&self, extension_id: &str) -> bool {
+        self.auto_install_extensions
+            .get(extension_id)
+            .copied()
+            .unwrap_or(true)
+    }
+
+    pub fn should_auto_update(&self, extension_id: &str) -> bool {
+        self.auto_update_extensions
+            .get(extension_id)
+            .copied()
+            .unwrap_or(true)
+    }
+}
+
+impl Settings for ExtensionSettings {
+    const KEY: Option<&'static str> = None;
+
+    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)
+                .chain(sources.server),
+        )
+    }
+}