vim-patch:9.0.1973: Clean up cmdline option completion code (#25477)

Problem:  Clean up cmdline option completion code
Solution: Fix various minor problems

- Fix manual array size calculations to just use `ARRAY_LENGTH()`.
- Fix unintentional typo in comments due to copy-paste error.
- Fix assert_equal() usages to pass the expected value to first
  parameter instead of 2nd one to avoid confusion.
- Fix signed vs unsigned warnings
- Correct misplaced comments about set_op_T and set_prefix_T
  and fix a typo in another comment

closes: vim/vim#13249
closes: vim/vim#13237

6d11347260

N/A patch:
vim-patch:9.0.1976: style: space before tab in optionstr.c

Co-authored-by: Yee Cheng Chin <ychin.git@gmail.com>

src/nvim/option.c

@@ -137,6 +137,7 @@ static char *p_vsts_nopaste;
 
 #define OPTION_COUNT ARRAY_SIZE(options)
 
+/// :set boolean option prefix
 typedef enum {
   PREFIX_NO = 0,  ///< "no" prefix
   PREFIX_NONE,    ///< no prefix