Issue Type: Feature Request

You can only pass an Uri or an object containing Uris to WebviewPanel.iconPath, while iconPath on QuickInputButton and TreeItem also supports ThemeIcon. This would increase codicon reusability.

VS Code version: Code 1.42.0 (ae08d54, 2020-02-06T10:51:34.058Z)
OS version: Windows_NT x64 10.0.18362