api: change nvim_command_output behavior

Implement nvim_command_output with `execute({cmd},"silent")`.

Behavior changes:
- does not provoke any hit-enter prompt
- no longer prepends a newline char
- does not capture some noise (like the "[New File]" message, see the
  change to tabnewentered_spec.lua)

Technically ("bug-for-bug") this a breaking change.  But the previous
behavior of nvim_command_output meant that it probably wasn't used for
anything outside of tests.

Also remove the undocumented `v:command_output` variable which was
a hack introduced only for the purposes of nvim_command_output.

closes #7726

src/nvim/eval.h

@@ -86,7 +86,6 @@ typedef enum {
     VV_OLDFILES,
     VV_WINDOWID,
     VV_PROGPATH,
-    VV_COMMAND_OUTPUT,
     VV_COMPLETED_ITEM,
     VV_OPTION_NEW,
     VV_OPTION_OLD,