Migrate keymap and settings + edit predictions rename (#23834)

- [x] snake case keymap properties
- [x] flatten actions
- [x] keymap migration + notfication
- [x] settings migration + notification
- [x] inline completions -> edit predictions 

### future: 
- keymap notification doesn't show up on start up, only on keymap save.
this is existing bug in zed, will be addressed in seperate PR.

Release Notes:

- Added a notification for deprecated settings and keymaps, allowing you
to migrate them with a single click. A backup of your existing keymap
and settings will be created in your home directory.
- Modified some keymap actions and settings for consistency.

---------

Co-authored-by: Piotr Osiewicz <piotr@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/editor/src/editor_tests.rs

@@ -1159,7 +1159,7 @@ fn test_fold_at_level(cx: &mut TestAppContext) {
     });
 
     _ = editor.update(cx, |editor, window, cx| {
-        editor.fold_at_level(&FoldAtLevel { level: 2 }, window, cx);
+        editor.fold_at_level(&FoldAtLevel(2), window, cx);
         assert_eq!(
             editor.display_text(cx),
             "
@@ -1183,7 +1183,7 @@ fn test_fold_at_level(cx: &mut TestAppContext) {
             .unindent(),
         );
 
-        editor.fold_at_level(&FoldAtLevel { level: 1 }, window, cx);
+        editor.fold_at_level(&FoldAtLevel(1), window, cx);
         assert_eq!(
             editor.display_text(cx),
             "
@@ -1198,7 +1198,7 @@ fn test_fold_at_level(cx: &mut TestAppContext) {
         );
 
         editor.unfold_all(&UnfoldAll, window, cx);
-        editor.fold_at_level(&FoldAtLevel { level: 0 }, window, cx);
+        editor.fold_at_level(&FoldAtLevel(0), window, cx);
         assert_eq!(
             editor.display_text(cx),
             "