feat(config): allow printing default and computed config

[davidkna]

Description

This PR adds starship print-config to print the computed config and starship print-config --default to print the default config.

I made a new struct in src/configs/mod.rs that contains all the module configs and implements serve::Serialize and ModuleConfig.
Options that have a None value need to be skipped because toml doesn't support None values and a warning comment about this is added at the top of the generated config files.
Before formatting as toml the struct needs to be converted into a toml::Value because otherwise the toml crate won't be able to properly handle printing the top-level root options.
I also got rid of the wrapper struct in custom.rs because the other modules don't use this for extensiosn, folders and extensions either.

Motivation and Context

Closes #2309
Related to #1921

Screenshots (if appropriate):

How Has This Been Tested?

• I have tested using MacOS
• I have tested using Linux
• I have tested using Windows

Checklist:

• I have updated the documentation accordingly. (not in-depth)
• I have updated the tests accordingly.

[davidkna]

I ended up having to modify the ModuleConfig macro to use default() to make printing custom modules possible, but I think this aligns better with the general expectations for its behaviour.

[andytom]

LGTM