feat(types): turn LocalForageWrapper into a generic class

[charpeni]

LocalForageWrapper is currently typed as string | object | null, but it solely depends on whether we serialize or not.

Also, localForage (and IndexedDB) supports storing more than string or object, it supports the following JS objects:

• Array
• ArrayBuffer
• Blob
• Float32Array
• Float64Array
• Int8Array
• Int16Array
• Int32Array
• Number
• Object
• Uint8Array
• Uint8ClampedArray
• Uint16Array
• Uint32Array
• String

I didn't want to introduce a breaking change by changing the type, so instead, I turned LocalForageWrapper into a generic class expecting a type for LocalForage and defaulting to string | object | null (current value).

It means that if you're storing binary data in IndexedDB, you can provide the type as such: LocalForageWrapper<Uint8Array>.

[charpeni]

I exported LocalForageInterface here as it could be useful.

I'm currently writing a wrapper for LocalForageWrapper that compresses the cache into a binary, so I need both the generic class and the interface here because I need to use composition over inheritance to be able to turn LocalForageWrapper into PersistentStorage<Uint8Array>, but make sure the newly created wrapper exposes string.