keymap_ui: Support unbinding non-user defined keybindings (#34318)

Closes #ISSUE

Makes it so that `KeymapFile::update_keybinding` treats removals of
bindings that weren't user-defined as creating a new binding to
`zed::NoAction`.


Release Notes:

- N/A *or* Added/Fixed/Improved ...

Cargo.lock

@@ -14677,6 +14677,7 @@ dependencies = [
  "schemars",
  "search",
  "serde",
+ "serde_json",
  "settings",
  "theme",
  "tree-sitter-json",