Settings: Add get_flags API for mapgen flags (mg_flags, mgv6_spflags, ...) (#9284)

Unified flags handling in C++ and Lua Settings API
     -> Reading only, for now. Writing can be implemented later, if needed.
API function to read the currently active flags
     -> was impossible from Lua

Co-authored-by: Wuzzy <wuzzy2@mail.ru>

src/script/lua_api/l_settings.h

@@ -42,6 +42,9 @@ class LuaSettings : public ModApiBase
 	// get_np_group(self, key) -> noiseparam
 	static int l_get_np_group(lua_State *L);
 
+	// get_flags(self, key) -> key/value table
+	static int l_get_flags(lua_State *L);
+
 	// set(self, key, value)
 	static int l_set(lua_State *L);