[RDY] Use lua generator in place of ex_cmds_defs header trick

This only removes header trick. Compared to #788 I did not add missing include and did not remove dependency on `ex_cmds_defs` in `vim.h`. `transform_ex_cmds_defs.pl` script works also for Vim `ex_cmds.h` file, must be removed in case we add any additional information to `ex_cmds.lua` or start adding our own commands.

Ref #379, #549, #788
`ex_cmds_enum.generated.h` is placed in `build/include` but `--help` message says that it should be placed in `build/src/nvim/auto`. I think it is the error in the help message.