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/ex_cmds/sign_spec.lua

@@ -16,8 +16,8 @@ describe('sign', function()
         nvim('command', 'sign place 34 line=3 name=Foo buffer='..buf2)
         -- now unplace without specifying a buffer
         nvim('command', 'sign unplace 34')
-        eq("\n--- Signs ---\n", nvim('command_output', 'sign place buffer='..buf1))
-        eq("\n--- Signs ---\n", nvim('command_output', 'sign place buffer='..buf2))
+        eq("--- Signs ---\n", nvim('command_output', 'sign place buffer='..buf1))
+        eq("--- Signs ---\n", nvim('command_output', 'sign place buffer='..buf2))
       end)
     end)
   end)