Make string to v3f parsing consistent, replace core.setting_get_pos() by core.settings:get_pos() (#15438)

Co-authored-by: sfan5 <sfan5@live.de>
Co-authored-by: Lars Müller <34514239+appgurueu@users.noreply.github.com>

src/util/string.h

@@ -20,6 +20,7 @@
 #include <cctype>
 #include <cwctype>
 #include <unordered_map>
+#include <optional>
 
 class Translations;
 
@@ -789,9 +790,9 @@ std::string sanitize_untrusted(std::string_view str, bool keep_escapes = true);
 void safe_print_string(std::ostream &os, std::string_view str);
 
 /**
- * Parses a string of form `(1, 2, 3)` to a v3f
+ * Parses a string of form `(1, 2, 3)` or `1, 2, 4` to a v3f
  *
  * @param str string
  * @return float vector
  */
-v3f str_to_v3f(std::string_view str);
+std::optional<v3f> str_to_v3f(std::string_view str);