Test Webview

[mjbvz]

• Windows @dbaeumer
• Linux @Tyriar
• MacOS - @octref

Complexity: 4

Test for #43713 and #28263

The webview api is now in stable. Read through the new docs (also opening bugs for anything confusing in the docs) and try building a simple extension that exercises the webview api. You can also find an existing extension that uses TextDocumentContentProvider with the previewHtml command, and try rewriting it to use a webview.

[mjbvz]

To test against the latest API do the following:

• open your extensions package.json file
• change the engine.vscode property to *
• run npm run postinstall (assuming you have the vscode module as dev-dependency)
• undo the engine-version change from step 2