feat: custom default config file

[Ark2000]

closes #173

Expected Behavior:

1. user://panku_config.cfg file exists, that is used as the config dictionary
2. If the user config file does not exist (for example, for first-time users), then use res://addons/panku_console/panku_default_config.cfg (which can be changed in project settings) as the config dictionary.
3. The plugin will always get a config dictionary, whether the config files exist or not.
4. If there's no corresponding key-value pair in the config dictionary, the default value in the function call will be used.

Note:
The user config file (which is located at user://) will always be created after the program starts, the default config file (located at res://) is in fact simply a read-only and for one use only template file.

[Ark2000]

CC @worron @TheContainer
Please take it a look.

[TheContainer]

CC @worron @TheContainer Please take it a look.

Works for me!

[worron]

Gave it brief testing and it works fine. There are some concerns inside get_config function, but it's rather cosmetic. Ready for merge overall, I think, pretty cool feature.