feat: ✨ Config JSON: Add API methods for saving data

[ithinkandicode]

NOTE: Depends on PR #143, which should be merged first.

Adds 3 methods to ModLoader. There's 2 for saving data, plus a small utility method for checking if a config JSON data object is valid.

# Utility
func is_mod_config_data_valid(config_obj: Dictionary) -> bool:

# Saving
func save_mod_config_dictionary(mod_id: String, data: Dictionary, update_config: bool = true) -> bool:
func save_mod_config_setting(mod_id: String, key:String, value, update_config: bool = true) -> bool:

Docs: https://github.com/GodotModding/godot-mod-loader/wiki/Upcoming-Features#config-json

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Closes #132:

• Config JSON: Add API methods for saving #132