Resolve "Classical logic" ✨️

Closes #78 🐔️👍️
Approved-by: belazy <aarthuur01@gmail.com>
Approved-by: aalbert <augustin.albert@bleu-azure.fr>

🦀️🍰🦀️🍰🦀️🍰

* chore(std): clarify separation of definitions across files + manual formatting

* chore: define std folder and reorganise madeleine files structure

* fixup: merge parts of contraposition and classical

Two new files bool.mdln and connectives.mdln containing True, False, And, Or, Not.

Also harmonized naming conventions: Prop and functions returning Prop
(like And) start with a capital letter, lemmas (whose type is itself a
Prop) are lowercased.

* feat: Resolve "Classical logic" (#78)

Add a classical.mdln file with three characterizations of excluded
middle.