feat: add snake_case into BUILTIN_MODULES

[sulabi]

Restore snake_case module names. Follow up of #1029

[MalpenZibo]

Thanks @sulabi , you spotted a real problem!

With #1029 alone, [appearance.modules.system_info] is silently ignored.

I'd rather fix it directly in #1029 though, because changing ModuleName has side effects I'd like to avoid:

• The deserializer is shared with the layout, so left = ["system_info"] would also become valid. We'd end up with two spellings for the same module everywhere.
• A custom module named like the snake_case form of a builtin (e.g. "tray") would be silently replaced by the builtin in the layout.

In the end I'd like to have this syntax:

[system_info]                      # module settings
[appearance.modules.system_info]   # module style: same name

Custom modules need their own namespace. In the settings a custom module can't clash with a builtin, because builtins have their own section and custom modules live in [[CustomModule]].
We should probably change this syntax also. For now, it's probably better to avoid breaking changes, so we should continue supporting the current syntax. In the end, I would like to have

[custom_modules.appLauncher]
command = "walker"

[appearance.modules.custom.appLauncher]
background = "primary"

So I think that we can close this PR and fix #1029. The custom module changes should go in a dedicated PR instead.

@romanstingler, what do you think?

[sulabi]

yeah, this works for me