Add SettingsSources::<T>::json_merge_with function (#10869)

This PR adds a `json_merge_with` function to `SettingsSources::<T>` to
allow JSON merging settings from custom sources.

This should help avoid repeating the actual merging logic when all that
needs to be customized is which sources are being respected.

Release Notes:

- N/A

crates/extension/src/extension_settings.rs

@@ -5,7 +5,6 @@ use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
 use settings::{Settings, SettingsSources};
 use std::sync::Arc;
-use util::merge_non_null_json_value_into;
 
 #[derive(Deserialize, Serialize, Debug, Default, Clone, JsonSchema)]
 pub struct ExtensionSettings {
@@ -42,10 +41,8 @@ impl Settings for ExtensionSettings {
     type FileContent = Self;
 
     fn load(sources: SettingsSources<Self::FileContent>, _cx: &mut AppContext) -> Result<Self> {
-        let mut merged = serde_json::Value::Null;
-        for value in [sources.default].into_iter().chain(sources.user) {
-            merge_non_null_json_value_into(serde_json::to_value(value).unwrap(), &mut merged);
-        }
-        Ok(serde_json::from_value(merged)?)
+        SettingsSources::<Self::FileContent>::json_merge_with(
+            [sources.default].into_iter().chain(sources.user),
+        )
     }
 }