[TME] Add two-step skid factor

[Nomagno]

Description

Currently, the skid-factor parameter in kart_characteristics.xml determines how much tyre degradation is multiplied during skidding. This will be changed to skid-factor-start and skid-factor-full to have different values for the first (pre-yellow sparks) and rest of stages, respectively. The motivation is purely that modders have expressed interest in having this possibility.