Vendor Catala LSP as a JS binary in VSCode extension

[denismerigoux]

For maximal compatibility, we have decided to vendor the Catala compiler and LSP as a JS artifact inside the VSCode extension. This issue tracks the implementation of this vendoring. The major issue is the polyfill in js_of_ocaml of all Unix-related functions. @AltGr says that the LearnOCaml app normally should have already found ways of doing this sort of things.