[whatwg/dom] Add Imperative Slot API (#966)

### This is a fresh PR, with updates from https://github.com/whatwg/dom/pull/860. I don't have permissions to update that PR, and the master-to-main transition made it difficult anyway.

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
  * In WPT: https://wpt.fyi/results/shadow-dom/slots-imperative-api-slotchange.tentative.html?label=master&label=experimental&aligned
* [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/966

-- Commit Summary --

  * Add updated imperative slot assignment API

-- File Changes --

    M dom.bs (42)

-- Patch Links --

https://github.com/whatwg/dom/pull/966.patch
https://github.com/whatwg/dom/pull/966.diff

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