Add welcome page icon

[xorye]

It seems VS Code 1.46 supports providing an icon for the webview tab: https://github.com/microsoft/vscode/blob/6af2987903c22bf9b19762d589bc46d4be636dc4/src/vs/vscode.d.ts#L6769

It would be nice if we had the Quarkus logo or something similar as the icon. Currently, the we have the default icon:

[fbricon]

Most likely we'd need a monochrome icon, that either works for dark and light themes, or 2 monochrome icons. Dunno if the Quarkus project has that kind of asset we could reuse. ping @maxandersen

[maxandersen]

https://design.jboss.org/quarkus/

There are at least a monochrome one.

[fbricon]

New icons: