settings: Flatten code actions formatters object (#39375)

Closes #ISSUE

Release Notes:

- settings: Changed code action format in `formatter` and
`format_on_save` settings.

**Previous format:**
```
{
  "code_actions": {
    "source.organizeImports": true,
    "source.fixAll": true
  }
}
```

**New format:**
```
[
  {"code_action": "source.organizeImports"},
  {"code_action": "source.fixAll"}
]
```

After #39246, code actions run sequentially in order. The structure now
reflects this and aligns with other formatter options (e.g., language
servers).

Both the `formatter` and `format_on_save` settings will be
auto-migrated.

Cargo.lock

@@ -9691,6 +9691,7 @@ dependencies = [
  "streaming-iterator",
  "tree-sitter",
  "tree-sitter-json",
+ "unindent",
  "workspace-hack",
 ]