Description of Change

Replaces the experimental BrowserView API with a new WebContentsView API.

• Documents the existing BaseWindow, which BrowserWindow extends.
  • Currently we duplicate all the documentation for the inherited methods into
    the BrowserWindow docs. Ideally we could deduplicate this, but for
    usability and consistency (avoiding "where did all the BrowserWindow
    methods go???" type questions), we should probably change the docs site to
    account for that change, e.g. to have the duplication of docs for inherited
    methods happen at render time.
• Adds one new BaseWindow read-write property, contentView.
• Documents the existing View and WebContentsView.
  • WebContentsView is the replacement for the old BrowserView class. We
    also expose and document the View class, because that is the type
    of BaseWindow.contentView, to which WebContentsViews are added.
• Adds several new View properties and methods.
  • removeChildView(), children, setBounds(), getBounds(), setBackgroundColor(), setVisible().
• Deprecates the BrowserView API and updates all references to it to point to WebContentsView instead.
  • We also need a new guide on how to use WebContentsView.
• Adds a JavaScript shim with the same API surface as the old BrowserView API, which
  under the hood delegates to the new WebContentsView API.
• Removes BrowserView-specific draggable region handling. This will be
  replaced with refactor: use views NonClientHitTest for draggable regions on mac #35603, but may need some more holes poked to enable draggable
  regions to work correctly on all platforms in WebContentsViews.
• Changes setAutoResize to behave consistently on all platforms. (The behavior on macOS has been changed to match the behavior on Windows.)

Still TODO:

• BrowserView had options for auto-sizing. This isn't yet possible with the
  currently exposed APIs of WebContentsView / View, but it should be
  doable! This is done, I ported the autoresizing logic from C++ to JS.
• There is some code about compositor recycling for BrowserViews that I glazed
  over and removed in this PR. I'm not really sure how to test "is this still
  working properly", but it deserves investigation. I tested this using the example from fix: Disable new fade animation for BrowserViews #14911 and it seems like things are okay (no flickering)!
• Test that background colors are working properly. as far as I can tell, they are.
• Test that onbeforeunload works properly in WebContentsView.
• Draggable regions are broken in WebContentsView. I think this should be fixed with chore: Move draggable regions implementation from NativeBrowserView into InspectableWebContentsView #35007, so I'm going to say that doesn't block this PR from landing.

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• tests are changed or added
• relevant documentation is changed or added
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: Added WebContentsView and BaseWindow, replacing the now-deprecated BrowserView APIs.