Problem: Generating nv_cmdidxs.h requires building Vim twice.
Solution: Move the table into a separate file and use a separate executable
to extract the command characters. (Ozaki Kiichi, closes#9669)
Problem: Generating the normal command table at runtime is inefficient.
Solution: Generate the table with a Vim script and put it in a header file.
(Yegappan Lakshmanan, closes#9648)