refactor: Remove self argument on *mod_main.gd* _init

[otDan]

Actually implement what is proposed by @KANAjetzt in #319 in a non breaking way

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Edit by @KANAjetzt:

• This PR removes and deprecates the self argument when initializing a mod's mod_main.gd script. Previously, it was used to call methods in the ModLoader class. However, with the introduction of v6.0.0, these methods have been relocated outside of ModLoader.

⚠️ Breaking Changes:

Please note that this change may break mods that still rely on the modLoader argument in their _init() function. To mitigate this, a fallback has been implemented that passes self as an argument if any arguments are detected in the mod_main.gd _init() function.

closes #318

[otDan]

also let me know a cleared way to tell the user to not use the argument as it's deprecated, currently it doesn't look right

[Qubus0]

yep that is safer