editor: Fix cursor_shape regression by not setting it to "bar" (#17934)

This fixes the regression introduced here:
https://github.com/zed-industries/zed/pull/17572#issuecomment-2355632615

Essentially: instead of always setting the value when saving settings,
we don't set it by default, but fall back to the default value if it's
not set.

That fixes Vim mode's cursor being overwritten when settings change.

Release Notes:

- N/A

assets/settings/default.json

@@ -121,8 +121,8 @@
   //  4. A box drawn around the following character
   //     "hollow"
   //
-  // Default: bar
-  "cursor_shape": "bar",
+  // Default: not set, defaults to "bar"
+  "cursor_shape": null,
   // How to highlight the current line in the editor.
   //
   // 1. Don't highlight the current line: