Add preview API

[jmcphers]

This change adds an API, service, and user interface for Positron's Preview panel.

The Preview API is modeled closely after VS Code's existing WebviewPanel API. Rather than try to co-opt that API to work in the Preview pane, which would have involved a lot of internal changes to VS Code's API to add new types and special cases, I've chosen to implement our own parallel API so we can discard the irrelevant options and add preview-specific logic.

Though we have our own PreviewPanel class, the actual WebView inside the panel that hosts the content is not a special Positron type; it is a standard VS Code WebView. This makes the PreviewPanel a sort of cousin of several other APIs that handle webviews.

graph TD
WebviewPanel --> WebView
WebviewView --> WebView
PreviewPanel --> WebView
WebView --> ExtHostWebView 
ExtHostWebView --> MainThreadWebView

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Because Positron's API and VS Code's API are completely separate, getting them to use the same backend for WebViews was challenging. This PR takes the (possibly controversial) approach of punching a hole in the RpcProtocol -- getRaw() -- that allows Positron's API to extract the actors created when VS Code's API initializes.

The user interface uses React, like our other view panes do, with hooks to repaint the pane when previews are opened/closed. Because you can't really attach/detach a WebView without losing content and state, we rely on VS Code's IOverlayWebview, which effectively paints the webview on top of an existing DOM element using absolute positioning. This, unfortunately, requires us to redraw the preview container every time its size or position changes.

Note that the Preview service lives in /contrib/ since it interacts heavily with webviews (which are also a contrib).

The Preview API includes some interactive demos you can run from the Zed console. Here are the new commands:

• preview open will open the Preview pane.
• preview show will reveal the Preview pane if it is currently closed.
• preview status will show the current status of the preview, including whether there is currently a preview and whether it is visible.
• preview close will close the preview if it's open.
• preview message will send a postMessage to the preview pane, which will appear in the pane itself.

[jjallaire]

I am not sufficiently conversant in the internals to really comment, but in any case LGTM.

After this is merged here are the outstanding would allow me to ship the Quarto extension to the marketplace w/ Positron support:

• Provide typing for global accessor of the Positron API for multi-IDE extensions #809
• executeCode API: Start language runtime associated with code if necessary #806
• Console: Send-to-console should not commingle with pending input #552