add custom json package for imports instead encoding/json support

[alimy]

• add custom json package for imports instead encoding/json support.

Usage:

//go:generate go-enum --marshal --sqlnullint --jsonpkg github.com/alimy/typst/json

/*
RunStatus
ENUM(

	None, // 未知
	Idle, // 空闲
	Pending, // 待命状态
	Processing, // 正在处理

)
*/
type RunStatus int

Will generate code like below:

package enum

import (
	"database/sql/driver"
	"errors"
	"fmt"
	"strconv"

	json "github.com/alimy/typst/json"
)

// other code
...

// MarshalJSON correctly serializes a NullRunStatus to JSON.
func (n NullRunStatus) MarshalJSON() ([]byte, error) {
	const nullStr = "null"
	if n.Valid {
		return json.Marshal(n.RunStatus)
	}
	return []byte(nullStr), nil
}

// UnmarshalJSON correctly deserializes a NullRunStatus from JSON.
func (n *NullRunStatus) UnmarshalJSON(b []byte) error {
	n.Set = true
	var x interface{}
	err := json.Unmarshal(b, &x)
	if err != nil {
		return err
	}
	err = n.Scan(x)
	return err
}