rationalize dialogs

[peterchinman]

As a precursor to making dialogs work well on mobile I have improved our general Dialog usage.

The proper usage of DialogWrapper is now:

<Dialog>
  <Dialog.Portal>
    <DialogWrapper>
      <div class="I-am-the-Content">

My philosophy here is:

1. Creating a Macro-themed Dialog should be simple.
2. Dialogs should handle content overflow gracefully.
3. The Content of a Dialog should not need to know that it is a Dialog. E.g. you should be able to move Dialog Content into a Drawer or Stand-alone Split, without changes. This means, e.g., that Dialog Content should, generally, not constrain its own height, but allow itself to be constrained by the Dialog Wrapper.

Changes

• DialogWrapper now handles all Dialog placement and frame-styling in a single call-site. (No more separate Dialog.Overlay, Dialog.Content, ClippedPanel, necessary.)
• Share Dialog and Compose Task Dialog are now properly constrained so that text input areas scroll on overflow.
• Introduced VerticalScrollIndicators component for decorative indication that a container is scrollable in scenarios in which we aren't using a scrollbar.

[github-actions]

Preview: https://peter-fix-dialog-heights-4ma9o7.preview.macro.com/app (6a1d979)