[RDY/RFC] 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
typo: "below"