Formatter for Catala code

[EmileRolley]

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As suggered in #252, adding a formatter to the Catala toolkit could be a great improvement for both the developer and the reviewer experience. Catala being a literate programming language, having a good readability of the code is essential.

Proposals

1. A first step could consist of the addition of a new Format backend using the surface AST of the compiler.
2. A second one could be done by implementing a standalone program using for instance Tree-sitter in order to take advantage of the incremental parsing.

[AltGr]

Additionally, this could allow automatic translation from one Catala dialect to another, e.g. French to Engish.

[denismerigoux]

Indeed, except for variable names...

[AltGr]

Update: it's still in progress, but I have a working tree-sitter parser at https://github.com/AltGr/tree-sitter-catala

and the included catala.el file contains an emacs mode (for emacs 29+) that can do automatic indentation based on it.

[AltGr]

The easiest way for formatting Catala at this point will be to use https://github.com/AltGr/tree-sitter-catala and https://github.com/tweag/topiary