Migrate LocalStorage

This plugin can be used in conjunction with
cordova-plugin-wkwebview-engine
to persist LocalStorage data when migrating from UIWebView to WKWebView. All related
files will be copied over automatically during startup so the user can simply pick up where they
left of.

How to use

Simply add the plugin to your cordova project via the cli:
``sh
cordoa plugin add cordova-plugin-migrate-localstorage
`

Notes

- LocalStorage files are only copied over once and only if no LocalStorage data exists for WKWebView
yet. This means that if you've run your app with WKWebView before this plugin will likely not work.
To test if data is migrated over correctly:
1. Delete the app from your emulator or device
2. Remove the cordova-plugin-wkwebview-engine and cordova-plugin-migrate-localstorage plugins
3. Run your app and store some data in LocalStorage
4. Add both plugins back
5. Run your app again. Your data should still be there!

- Once the data is copied over, it is not being synced back to UIWebView so any changes done in
WKWebView will not persist should you ever move back to UIWebView. If you have a problem with this,
let us know in the issues section!

Background

One of the drawbacks of migrating Cordova apps to WKWebView` is that LocalStorage data does
not persist between the two. Unfortunately,
cordova-plugin-wkwebview-engine
does not offer a solution for this out of the box (see
https://issues.apache.org/jira/browse/CB-11974?jql=project%20%3D%20CB%20AND%20labels%20%3D%20wkwebview-known-issues).