fix!: preserve type of string values when storing

[molvqingtai]

Before the fix:

await setItem('foo', '[]')
await getItem('foo') // []

After the fix:

await setItem('foo', '[]')
await getItem('foo') // '[]'

This will fix: https://github.com/unjs/unstorage/issues/277

[pi0]

Thanks for PR. As noticed it makes lots of tests fail since it is a breaking behavior change.

I will consider it for unstorage v2 👍🏼