Add CLI option to run without autoloads.

[Jack-023]

When running scripts from the command-line, it would be helpful to be able to pass a flag to ignore autoloads in the project (eg --ignore-autoloads or --disable-autoloads). This would be helpful because some addons / tools are more like shell scripts written in gdscript, and break otherwise safe assumptions about state that autoloaded scripts may make. This is particularly relevant in the context of tools that are intended to be run in headless mode.