Draft: jsoo interface

[maxtori]

Checklist

If this PR adds a feature or has breaking changes

• Complete or update the documentation for the feature on the Catala book at https://github.com/CatalaLang/catala-book.
• The corresponding PR is:
  • Update the existing Catala code in case of breaking changes at https://github.com/CatalaLang/catala-examples.
  • The corresponding PR is:
  • Update the Catala language server in case of breaking changes at https://github.com/CatalaLang/catala-language-server
• The corresponding PR is:
  Adding a js_of_ocaml backend via interface files from ocaml backend

If this PR contains syntax changes

I confirm that have have checked and updated each of the following items if this PR impacts them:

• Syntax cheat sheet at doc/syntax/syntax_*.catala_* and doc/syntax/catala_*.typ.
• Syntax highlighting plugins in syntax_highlighting/ (all of them).
• Syntax constructions in the tree-sitter grammar:
  • Base grammar: https://github.com/CatalaLang/tree-sitter-catala/blob/master/grammar.js
    • The corresponding PR is:
  • Formatting spec: https://github.com/CatalaLang/catala-format/blob/master/catala.scm
    • The corresponding PR is:

[AltGr]

I have a thing against redundant names, so I'd prefer File.with_extension to File.file_with_extension ;)