Add completion for nested settings (#3183)

Co-authored-by: Michael Davis <mcarsondavis@gmail.com>

helix-term/src/ui/mod.rs

@@ -283,14 +283,28 @@ pub mod completers {
         names
     }
 
+    /// Recursive function to get all keys from this value and add them to vec
+    fn get_keys(value: &serde_json::Value, vec: &mut Vec<String>, scope: Option<&str>) {
+        if let Some(map) = value.as_object() {
+            for (key, value) in map.iter() {
+                let key = match scope {
+                    Some(scope) => format!("{}.{}", scope, key),
+                    None => key.clone(),
+                };
+                get_keys(value, vec, Some(&key));
+                if !value.is_object() {
+                    vec.push(key);
+                }
+            }
+        }
+    }
+
     pub fn setting(_editor: &Editor, input: &str) -> Vec<Completion> {
         static KEYS: Lazy<Vec<String>> = Lazy::new(|| {
-            serde_json::json!(Config::default())
-                .as_object()
-                .unwrap()
-                .keys()
-                .cloned()
-                .collect()
+            let mut keys = Vec::new();
+            let json = serde_json::json!(Config::default());
+            get_keys(&json, &mut keys, None);
+            keys
         });
 
         let matcher = Matcher::default();