Show update spinner by default in `dev_build=yes` editor builds

[Calinou]

This ensures contributors can see when something forces the editor to redraw constantly.

The existing boolean true value will be casted to 1 in the setting, so it'll switch to Enabled automatically if the setting was previously enabled. If the setting is disabled, it'll default to Auto (enabled on dev builds, disabled otherwise).

• See https://github.com/godotengine/godot/pull/88084.