[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
> AFAIR I do not have non-integer keys or nil values. It will only be risky if we have enough commands to experience precision loss. If you see a place where this is not true please point it.

When we have more than 2^53 commands I suppose we'll have bigger problems. Perhaps a comment above the table itself to not leave gaps or have nil values.