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

test/functional/ui/cmdline_highlight_spec.lua

@@ -849,7 +849,7 @@ describe('Ex commands coloring support', function()
       {EOB:~                                       }|
                                               |
     ]])
-    eq('\nError detected while processing :\nE605: Exception not caught: 42',
+    eq('Error detected while processing :\nE605: Exception not caught: 42',
        meths.command_output('messages'))
   end)
   it('errors out when failing to get callback', function()