Allow `.` in fields to reduce nesting

[RaphGL]

Just like in TOML, allow writing:

editor = {
    theme = {
        dracula = {
           foreground = "black"
           background = "white"
        }
    }
}

like this as well:

editor.theme.dracula = {
    foreground = "black"
    background = "white"
}

While TOML's lack of indentation is a negative, this nesting syntax is actually good to prevent too much nesting on more complex config files.