Refactor the building process

[thibautbenjamin]

Various things are a bit ugly in the current build process of catt and the CI.

This branch is there to:

• [X] clean up the flake file
• [X] cleanup and update the various dune files
• [X] make the CI use the flake definition to build
• [X] make the CI use the flake checks to check
• [X] clean up the tests: remove useless and redundant ones, merge some others, add a test with --no-builtins
• [x] add tests where the checking fails: always catch the exceptions and add a --keep-going option so that catt it keeps going after encountering an error
• [x] add linting and code to the checks of the flake
• [x] add a separate package in the flake for the catt-mode emacs package, and move catt-mode.el into share/site-lisp
• [x] add a separate package in the flake to define the catt-web package
• [x] make the CI use the flake to build the catt-web package and publish that on the correct branch
• ~[ ] define a tree-sitter syntax for catt and make a catt-ts-mode for emacs~

[thibautbenjamin]

BLOCKED. This PR looks great but reformats all of the codebase at a time where there are a barillion open branches. This needs to be done more cleanly, ideally once fewer feature branches are open

Seen offline: I do not believe we will really find a time with less PR's than now, especially because the vast majority of them introduce new code and will not cause conflict. Also I think conflicts over reformatting should be easy to merge. I will thus rebase all open PR's onto this branch and proceed with the merge

[thibautbenjamin]

I rebased all the open branches on this one, merging now!