export renderSettled via @ember/renderer package

[cafreeman]

Serves as the first step in the implementation of https://github.com/cafreeman/rfcs/blob/dont-use-this-for-template-values-in-tests/text/0785-remove-set-get-in-tests.md

This PR simply exposes an existing utility (which is already fully tested) in a public package so that it can be consumed in ember-test-helpers.

[jacobq]

No fair! You got github item number 20000, you lucky 😆 (I got #20001 though)

[cafreeman]

@jacobq my lucky day :)

[chriskrycho]

In case anyone else is wondering, I had to refresh myself on the RFC. We are indeed introducing a new top-level module here:

In order to implement rerender, we would also expose the work done by @rwjblue on renderSettled as public API in a new @ember/renderer module so that it could then be imported and used by @ember/test-helpers without issuue [sic].

[chriskrycho]

Thank you, @cafreeman!

[chriskrycho]

Whoops! This should not have been merged yet, as we haven’t merged the RFC and it still needs the feature flag to go with it. We’ll get that later but sorted, probably early next week.

[kategengler]

@chriskrycho Branch point should be next Monday (21st), so the flag is needed by then

[chriskrycho]

cc @cafreeman