[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
Looks like `src/nvim build/include` is the source and `build/src/nvim/auto` is the target:

    lua genex_cmds.lua src/nvim build/include build/src/nvim/auto