Detection of the [TSUP]3[/TSUP][ITAL]P[/ITAL][TINF]2[/TINF] → [TSUP]3[/TSUP][ITAL]P[/ITAL][TINF]1[/TINF] Submillimeter Transition of [TSUP]13[/TSUP]C [CSC]i[/CSC] in the Interstellar Medium: Implication for Chemical Fractionation