Refactor language.rs

[o2sh]

Currently, adding support for a new language requires modifying the language.rs at multiple locations.
This process can be quite cumbersome and error prone (especially for new contributors).

Some shortcuts can be taken:

• Assuming the ascii file to be in "../resources/language.ascii" => match case in get_ascii_art() no more needed
• Using tokei::LanguageType instead of our own Enum, else (if not possible) assume that the name matches => match case in fn from(language: tokei::LanguageType) no more needed.
• ...

[spenserblack]

We might be able use a macro for most language definition, since it's fairly repetitive code. This macro could also define tests.