[whatwg/dom] Add Imperative Slot API (#860)

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The explainer for this feature is here: https://github.com/w3c/webcomponents/blob/gh-pages/proposals/Imperative-Shadow-DOM-Distribution-API.md  
The issue discussion is here: [3534](https://github.com/whatwg/html/issues/3534)  
There is a corresponding [Pull Request](https://github.com/whatwg/html/pull/5483) for the HTML spec that goes along with this PR.  

- [ ] At least two implementers are interested (and none opposed):  
   * I'm working to get this implemented in Chromium, tracking bug [here](http://crbug.com/869308).
   * The implementation is based on what was agreed in the last [F2F Meeting](https://github.com/whatwg/html/issues/3534#issuecomment-537802687).

- [x] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
   * In WPT: https://wpt.fyi/results/shadow-dom/slots-imperative-slot-api.tentative.html?label=master&label=experimental&aligned&q=slots-imperative-slot-api.tentative.html
   * More tests are being implemented, but the basics are there.

- [x] [Implementation bugs](https://github.com/whatwg/meta/blob/master/MAINTAINERS.md#handling-pull-requests) are filed:
   * Chrome: https://crbug.com/869308
   * Firefox: None yet
   * Safari: None yet

You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/dom/pull/860

-- Commit Summary --

  * Add Imperative Slot API
  * Update slot assignment default wording

-- File Changes --

    M dom.bs (20)

-- Patch Links --

https://github.com/whatwg/dom/pull/860.patch
https://github.com/whatwg/dom/pull/860.diff

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