Provide Proper Migration Documentation

[Confectrician]

While starting this in #247 i realized that it would be more sufficient to provide a proper migration logic in place for future releases.

We should:

• Add some logic to recognize when the openhab extension got installed/updated
• Check for major version changes
• Provide a WebView view with detailed instructions for the current update
• Display this migration path view
• Provide migration helpers where possible
• Store relevant extension date needed to determine this in globalstate (https://code.visualstudio.com/updates/v1_31#_global-storage-path) (https://stackoverflow.com/questions/54039837/where-to-store-global-binary-data-for-a-vscode-extension)