String and math fixes

- Added missing static String constructors
- Implemented String operator for math types
- Added XYZ and YXZ euler angles methods
- Fixed wrong det checks in Basis
- Fixed operator Quat in Basis

src/core/Vector2.cpp

@@ -252,7 +252,7 @@ Vector2 Vector2::snapped(const Vector2& p_by) const
 
 Vector2::operator String() const
 {
-	return String(); /* @Todo String::num() */
+	return String::num(x) + ", " + String::num(y);
 }