Add minetest.is_valid_player_name utility

src/player.cpp

@@ -30,6 +30,10 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "porting.h"  // strlcpy
 
 
+bool is_valid_player_name(std::string_view name) {
+	return !name.empty() && name.size() <= PLAYERNAME_SIZE && string_allowed(name, PLAYERNAME_ALLOWED_CHARS);
+}
+
 Player::Player(const std::string &name, IItemDefManager *idef):
 	inventory(idef)
 {