[popover2] feat: Popover2 and Tooltip2 components

[adidahiya]

Fixes #4023

Checklist

• Includes tests
• Update documentation

Changes proposed in this pull request:

• Creates a new package @blueprintjs/popover2
  • This package depends on popper.js v2 and react 16.8+
  • This package contains ports of Popover and Tooltip to use the new version of the positioning library, with minimal changes to the API
• Notable changes compared to <Popover>:
  • There is no more .bp3-popover-wrapper wrapper element surrounding the target and overlay. Both the target and overlay are rendered out to the virtual DOM as siblings.
  • There is an option to no longer render a .bp3-popover-target wrapper element surrounding the target as well. If renderTarget prop is supplied, its return value will be rendered out directly to the DOM without a wrapper.
    • Note: in this case it is the consumer's responsibility to disable a <Tooltip2> target which is a direct child of a <Popover2> when the popover is open (<Popover> would handle this automatically for you, with its wrapper element behavior).
  • The old API of supplying a target as the first child of <Popover2> is retained. In this case, a .bp3-popover-target wrapper element will be generated around the target child, so we can atttach event handlers to it.
  • For certain prop configurations (openOnTargetFocus={true} and hover interaction kinds), a tabIndex was previously generated and applied to the cloned props.children element directly. Now, that tabIndex is applied to the bp3-popover2-target wrapper element to increase the chance of this feature working in more situations.

Reviewers should focus on:

• DX of the new renderTarget API

Screenshot

(pretty much identical to Popover)

Not addressed in this PR

• eventually we should move to jest + puppeteer for unit testing

[blueprint-bot]

core: fix lint

Previews: documentation | landing | table

[adidahiya]

self-review

[blueprint-bot]

fix lint

Previews: documentation | landing | table

[blueprint-bot]

fix more lint

Previews: documentation | landing | table

[trixn86]

Not addressed in this PR

• eventually we should move to jest + puppeteer for unit testing

Have you considered using cypress.io for end-to-end/integration tests? In my (personal) opinion this library has many benefits over jest + selenium. The test runner itself is a free open source library and I haven't had so much fun writing end-to-end test with another tool yet. It's very fast and intuitive. They also provide a dashboard service that has a free plan for open source projects.

[adidahiya]

@trixn86 I've looked at cypress but not used it myself yet. We need fast unit tests in Blueprint, so I don't think we can fully switch over to an ETE test suite and retain the kind of developer experience we currently have in this repo. I imagine that spinning up all the services necessary for ETE tests is slower than mocha+karma or jest+puppeteer, but if I'm wrong about that do let me know.