Use make_array to avoid specifying the array size

When I wrote this line I wanted to avoid adding the array size but
I didn't know about make_array().

I had unsuccessfully tried some alternatives, for example

	Array{"a", "b", "c"}

which doesn't work because we need StringView (c.f. git blame on
this line)

also

	Array<StringView>{"a", "b", "c"}

doesn't work because it's missing a template argument.

src/commands.cc

@@ -1452,7 +1452,7 @@ KeymapMode parse_keymap_mode(StringView str, const KeymapManager::UserModeList&
     return (KeymapMode)(std::distance(user_modes.begin(), it) + offset);
 }
 
-static constexpr Array<StringView, 8> modes = { "normal", "insert", "menu", "prompt", "goto", "view", "user", "object" };
+static constexpr auto modes = make_array<StringView>({ "normal", "insert", "menu", "prompt", "goto", "view", "user", "object" });
 
 const CommandDesc debug_cmd = {
     "debug",