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/picker/src/picker.rs

@@ -26,6 +26,7 @@ actions!(picker, [ConfirmCompletion]);
 /// ConfirmInput is an alternative editor action which - instead of selecting active picker entry - treats pickers editor input literally,
 /// performing some kind of action on it.
 #[derive(Clone, PartialEq, Deserialize, JsonSchema, Default)]
+#[serde(deny_unknown_fields)]
 pub struct ConfirmInput {
     pub secondary: bool,
 }