Adding file for larex preamble file

[NiraliJoshi17]

Adding a preamble file as a support-files.tex and define a rule for this in dune file.

[jonludlam]

Hi @NiraliJoshi17, thanks for this! It's looking good so far, you've added the latex file as an OCaml module. The next steps are to install it as part of the support-files CLI sub command, as suggested in issue #588

Also you'll need to reformat the dune file as it's causing the CI to fail. To do this, install ocamlformat.0.19.0 and run

dune build @fmt 
dune promote

and commit the results.

[EmileTrotignon]

Is this still relevant ? If yes it should be reasonably easy to rebase and merge, so I am including it in the 2.4 milestone for now. @NiraliJoshi17

[jonludlam]

The feature is still needed, but the implementation would be different today (crunch), and some futher work is required for proper integration.

[jonludlam]

I'm going to close this for now. When we do a support-files like command for latex we can always come back to take a look at this PR.