[WIP] VimL to lua translator

This is the current work on VimL to lua translator.

TODO:

- [x] expression parser
- [ ] expression parser tests
- [ ] expression parser documentation
- [ ] Ex commands parser
- [ ] Ex commands parser tests
- [ ] Ex commands parser documentation
- [ ] VimL to lua translator
- [ ] VimL to lua translator tests
- [ ] VimL to lua translator documentation
- [ ] vim/neovim incompatibilities documentation