Add emacs plugin
The first version of Emacs plugin for LIGO. It features three major modes (ligo-pascal-mode, ligo-caml-mode, and ligo-reason-mode). A recommended way to launch one of the modes is using ligo-mode function that autodetects the right mode and sets everything up (e.g., it checks whether lsp-mode is installed and configures the language client).
Known issues:
- ReasonLIGO mode sometimes fails to highlight type annotations
- The language server should be installed separately