Allow creation of discriminated unions with a readonly array of options

[haines]

If I extract the array of options out of the z.discriminatedUnion call, type-checking fails because an array is not assignable to a non-empty tuple:

const options = [
  z.object({ type: z.literal("x"), val: z.literal(1) }),
  z.object({ type: z.literal("y"), val: z.literal(2) }),
];

const schema = z.discriminatedUnion("type", options);

TS2345: Argument of type '(ZodObject<{ type: ZodLiteral<"x">; val: ZodLiteral<1>; }, "strip", ZodTypeAny, { type: "x"; val: 1; }, { type: "x"; val: 1; }> | ZodObject<{ type: ZodLiteral<...>; val: ZodLiteral<...>; }, "strip", ZodTypeAny, { ...; }, { ...; }>)[]' is not assignable to parameter of type '[ZodDiscriminatedUnionOption<"type">, ...ZodDiscriminatedUnionOption<"type">[]]'.
  Source provides no match for required element at position 0 in target.

However, if I add as const to the options definition to make it a readonly tuple, now type-checking fails because the options are not mutable:

TS2345: Argument of type 'readonly [ZodObject<{ type: ZodLiteral<"x">; val: ZodLiteral<1>; }, "strip", ZodTypeAny, { type: "x"; val: 1; }, { type: "x"; val: 1; }>, ZodObject<{ type: ZodLiteral<...>; val: ZodLiteral<...>; }, "strip", ZodTypeAny, { ...; }, { ...; }>]' is not assignable to parameter of type '[ZodDiscriminatedUnionOption<"type">, ...ZodDiscriminatedUnionOption<"type">[]]'.
  The type 'readonly [ZodObject<{ type: ZodLiteral<"x">; val: ZodLiteral<1>; }, "strip", ZodTypeAny, { type: "x"; val: 1; }, { type: "x"; val: 1; }>, ZodObject<{ type: ZodLiteral<...>; val: ZodLiteral<...>; }, "strip", ZodTypeAny, { ...; }, { ...; }>]' is 'readonly' and cannot be assigned to the mutable type '[ZodDiscriminatedUnionOption<"type">, ...ZodDiscriminatedUnionOption<"type">[]]'.

This PR makes it possible to do this with as const by making the generic constraint readonly.

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[haines]

@colinhacks could you please take a look?