[v5] [popover2] fix(Popover2): restore basic support for some legacy class names

[adidahiya]

Changes proposed in this pull request:

Fix a subtle bug in { Popover2 } from "@blueprintjs/popover2".

Previously, this export was a simple alias for { Popover } from "@blueprintjs/core". When upgrading code bases to Blueprint v5, I found a some custom styles which reference CSS selectors such as:

#{bp.$ns}-popover2-target { ... }
#{bp.$ns}-popover2 { ... }
#{bp.$ns}-popover2-content { ... }
#{bp.$ns}-popover2-open { ... }
#{bp.$ns}-popover2-arrow-fill { ... }

<Popover> no longer renders those class names, which constitutes a break in the CSS API. We expect some breaks, but it's also possible to mitigate them by restoring some of the popover2 classes in <Popover2>. We can't reasonably add back all of them, but we can cover the most used one: the target element class.

<Popover2 />
// will now render:
<Popover className="bp5-popover-target" popoverClassName="bp5-popover" />

[adidahiya]

[v5] [popover2] fix(Popover2): restore basic support for some legacy class names

Build artifact links for this commit: documentation | landing | table | demo

This is an automated comment from the deploy-preview CircleCI job.