[Feature]: allow themes to modify the site configuration

[NTBBloodbath]

The title is very general but I didn't want to make it so long. The idea is to allow themes to extend norgolith.toml when installed through their theme.toml. This would allow a theme to easily provide variables and settings to its users.

For example, if the norgolith-pico-theme lets you configure the PicoCSS version through a pico_version variable then it should be able to extend the user's norgolith.toml to add it.

# theme.toml, variables get converted to theme
[variables]
pico_version = "X.Y.Z"

# norgolith.toml
[theme]
pico_version = "X.Y.Z"

This way, the user would not have to modify any values in the theme directory, and that conversion would happen directly in the installation/update process. This is just an idea, it may not be carried out or implemented exactly like this, I documented it so I don't forget it. All feedback is welcome.