[Plan Item] Propose new Webview API

[mjbvz]

Follow up on #41047 which explored improving the webview API. Part of #28263

This iteration we'd like to move the new API to be a fully documented proposed API and also self-host the markdown extension on the new API

[mjbvz]

Initial version of API taken into proposed with #42690

[mjbvz]

Proposed API changes for next iteration:

• Add a Webview.viewColum property: readonly viewColumn?: ViewColumn;.

• Rename WebviewOptions.keepAlive to make it more clear what this property actually does

• Rename Webview.onMessage per Better UI extensibility: retain webviews, provide communication channel #28263 (comment)

• Move from Webview.onDidBecomeActive/Webview.onDidBecomeInactive to an API closer to window.onDidChangeActiveTextEditor. Current proposal: window.onDidChangeActiveWebview. Would be fired with undefined when a user switches to a normal text editor. Also would be fired when a webview changes columns.

// cc @ivankravets @eamodio @pavelfeldman