Warn webview extensions that use the vscode-resource: scheme directly

[mjbvz]

Problem

The webview.asWebviewUri is now the preferred way to load local resources in webview. We really want all webviews to migrate to using this api instead of using vscode-resource uri directly. Reasons:

• asWebviewUri (mostly) handles some tricky edge cases around network drives on windows
• asWebviewUri makes it possible to have a webviews that can load a mix of local and remote content.
• asWebviewUri work much better on the web

Proposal

When developing a webview extension, warn in the console if that extension tries creating a webview that looks like it uses vscode-resource instead of webview.asWebviewUri

[mjbvz]

For verification, use the cat coding webview sample: https://github.com/Microsoft/vscode-extension-samples/tree/master/webview-sample

Specifically, roll back this change for testing: microsoft/vscode-extension-samples@708bc30#diff-90272a6a871150e841f43e808eccbd84

Test

1. In development mode, you get a debug console warning when using vscode-resource in the html (such as for loading an image)
2. Adding any calls to .asWebviewUri suppress the warning
3. The warning does not show up in non-development mode

[mjbvz]

Note that our check for vscode-resource usage in the html is intentionally pretty simple and easy to get around. It uses the same simple logic we use to rewrite uris for the web but still should catch many common use cases of vscode-resource