[core] feat(Portal): Add portalContainer context option

[braeden]

Fixes #6210

Checklist

• Includes tests
• Update documentation

Changes proposed in this pull request:

This changes forces all (blueprint) children that who render below a given PortalProvider (like <Dialog/>, <Popover2/>) to have a common element where React.createPortal usages become direct children of.

To be more precise here, portalContainer acts ONLY as a parent element of a new div and that new div becomes the target container in the React.createPortal syntax -- so children are rendered into that element.

We also memoize the PortalProvider value to avoid thrashing on every re-render.

Reviewers should focus on:

• I changed the way that addStopPropagationListeners, removeStopPropagationListeners (and maybeAddClass) work -- using React.useEffect's cleanup function to avoid leaving, what I believe would have been, dangling listeners when the portal unmounted.
• I removed the hasMounted state, since it was purely duplicative with portalElement != null
• I removed Portal.defaultProps and replaces it with an internal ?? document?.body fallback.

Screenshot

[adidahiya]

looks good! just need a few documentation fixes

[adidahiya]

nice, thanks for fixing this dependency list

[adidahiya]

Another docs suggestion: let's update the link to React context docs. This line:

blueprint/packages/core/src/components/portal/portal.md

Line 31 in 8623da5

__Portal__ supports some customization through [React context](https://reactjs.org/docs/context.html).

Should instead point to: https://react.dev/learn/passing-data-deeply-with-context

[adidahiya]

thanks @braeden 👍🏽