Implement attachement of code comments to AST node via an AST attribute system

[denismerigoux]

In order to make progress on #673, the attribute system of #392 has entered the critical path. This issue states that work on the attribute system should begin with the following objectives :

• capture comments inside Catala code blocks as either surfact AST nodes or annotations on neighbouring surface AST nodes (the parser and lexer should be modified) ;
• the rules for knowing where to attach comments to AST nodes can be simple at first : the comment is always attached to the AST node of the token immediately after the comment in the linear flow of tokens ;
• code comments attached to AST nodes should be pushed down the compilation chain at least to Desugared (used by catala-language-server: test case format should include prose sections and possibly metadata #673).