Future-proof indent guides settings for panels (#19878)

This PR ensures that we do not have to break the indent guides settings
for the project/outline panel. In the future we might want to have a
more granular way to control when to show indent guides, or control
other indent guide properties, like its width.

Release Notes:

- N/A

assets/settings/default.json

@@ -346,8 +346,6 @@
     "git_status": true,
     // Amount of indentation for nested items.
     "indent_size": 20,
-    // Whether to show indent guides in the project panel.
-    "indent_guides": true,
     // Whether to reveal it in the project panel automatically,
     // when a corresponding project entry becomes active.
     // Gitignored entries are never auto revealed.
@@ -371,6 +369,17 @@
       /// 5. Never show the scrollbar:
       ///    "never"
       "show": null
+    },
+    // Settings related to indent guides in the project panel.
+    "indent_guides": {
+      // When to show indent guides in the project panel.
+      // This setting can take two values:
+      //
+      // 1. Always show indent guides:
+      //    "always"
+      // 2. Never show indent guides:
+      //    "never"
+      "show": "always"
     }
   },
   "outline_panel": {
@@ -388,15 +397,24 @@
     "git_status": true,
     // Amount of indentation for nested items.
     "indent_size": 20,
-    // Whether to show indent guides in the outline panel.
-    "indent_guides": true,
     // Whether to reveal it in the outline panel automatically,
     // when a corresponding outline entry becomes active.
     // Gitignored entries are never auto revealed.
     "auto_reveal_entries": true,
     /// Whether to fold directories automatically
     /// when a directory has only one directory inside.
-    "auto_fold_dirs": true
+    "auto_fold_dirs": true,
+    // Settings related to indent guides in the outline panel.
+    "indent_guides": {
+      // When to show indent guides in the outline panel.
+      // This setting can take two values:
+      //
+      // 1. Always show indent guides:
+      //    "always"
+      // 2. Never show indent guides:
+      //    "never"
+      "show": "always"
+    }
   },
   "collaboration_panel": {
     // Whether to show the collaboration panel button in the status bar.