Remove vscode-webview-ui-toolkit dependency

[noeldevelops]

https://github.com/microsoft/vscode-webview-ui-toolkit has been deprecated and is no longer supported. We've migrated webview forms to HTML & CSS until a need for a components library arises. We still use a few components from the package in message-viewer.html and we should migrate them to HTML asap

[bendera]

If simple forms are enough, I recommend this to your attention:
https://vscode-elements.github.io/elements-lite/