added z.jsonString() schema primitive

[Sachinkry]

Implements the z.jsonString() schema primitive for validating and parsing JSON strings, addressing issue #4646.

Basic JSON Syntax Validation

const schema = z.jsonString();
schema.parse('{"name": "Alice", "age": 30}'); // ✅ returns original string
schema.parse("invalid json"); // ❌ throws error

JSON Parsing with Schema Validation

const userSchema = z.jsonString(z.object({
  name: z.string(),
  age: z.number(),
}));
userSchema.parse('{"name": "Alice", "age": 30}'); // ✅ returns { name: "Alice", age: 30 }

Details

• Implement z.jsonString() to validate JSON strings and optionally parse+validate against an inner schema.
• Adds core $ZodJSONString, public APIs (classic/mini), and pipe-based parsing to inner schemas.
• Emits invalid_format on JSON syntax errors; returns original string when no inner schema is provided.
• Includes focused tests (10) and documentation updates (API + v4 changelog).