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/workspace/src/pane_group.rs

@@ -725,6 +725,7 @@ impl PaneAxis {
 }
 
 #[derive(Clone, Copy, Debug, Deserialize, PartialEq, JsonSchema)]
+#[serde(rename_all = "snake_case")]
 pub enum SplitDirection {
     Up,
     Down,
@@ -807,14 +808,6 @@ impl SplitDirection {
     }
 }
 
-#[derive(Clone, Copy, Debug, Deserialize, JsonSchema, PartialEq)]
-pub enum ResizeIntent {
-    Lengthen,
-    Shorten,
-    Widen,
-    Narrow,
-}
-
 mod element {
     use std::mem;
     use std::{cell::RefCell, iter, rc::Rc, sync::Arc};