fix bug

1. fix bug about base64 string to arraybuffer. It should decode base64 string to utf8 string at first, and then denpend on utf8 standard to transform utf8 string to arraybuffer. That way is similar to nodejs.

support entire docs

1. support readFile by openning a system dailog
2. support writeFile without backend
3. support transfom an ArrayBuffer into a string
4. support transfom a string into an ArrayBuffer