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/helpers.lua

@@ -644,7 +644,7 @@ local function alter_slashes(obj)
 end
 
 local function hexdump(str)
-  local len = string.len( str )
+  local len = string.len(str)
   local dump = ""
   local hex = ""
   local asc = ""
@@ -652,22 +652,20 @@ local function hexdump(str)
   for i = 1, len do
     if 1 == i % 8 then
       dump = dump .. hex .. asc .. "\n"
-      hex = string.format( "%04x: ", i - 1 )
+      hex = string.format("%04x: ", i - 1)
       asc = ""
     end
 
-    local ord = string.byte( str, i )
-    hex = hex .. string.format( "%02x ", ord )
+    local ord = string.byte(str, i)
+    hex = hex .. string.format("%02x ", ord)
     if ord >= 32 and ord <= 126 then
-      asc = asc .. string.char( ord )
+      asc = asc .. string.char(ord)
     else
       asc = asc .. "."
     end
   end
 
-  return dump .. hex
-  .. string.rep( "   ", 8 - len % 8 ) .. asc
-
+  return dump .. hex .. string.rep("   ", 8 - len % 8) .. asc
 end
 
 local module = {