fix: Fix the wrong type of stringify() (#153)

Author: kuu

types.ts

@@ -520,6 +520,6 @@ export type TagParam =
 export type UserAttribute = number | string | Uint8Array;
 
 export type PostProcess = {
-  segmentProcessor: ((lines: string[], start: number, end: number, segment: Segment, i: number) => void) | undefined;
-  variantProcessor: ((lines: string[], start: number, end: number, variant: Variant, i: number) => void) | undefined;
+  segmentProcessor?: ((lines: string[], start: number, end: number, segment: Segment, i: number) => void);
+  variantProcessor?: ((lines: string[], start: number, end: number, variant: Variant, i: number) => void);
 };