[w3c/selection-api] Fire selectionchange on input/textarea elements (#141)

Closes (partially) #53, since this only covers `selectionchange` without `selectstart` because of no implementation.

This follows [the suggestion from @smaug----](https://github.com/w3c/selection-api/issues/53#issuecomment-692672383) - to bubble up from text controls to `document`, since it allows to easily detect which text control fired it.

([`document.activeElement` can be used in some cases](https://github.com/w3c/selection-api/issues/53#issuecomment-677500484) to detect the source, but it's not always useful since it can be fired without focusing, e.g. through `.setSelectionRange()`.)

The following tasks have been completed:

 * [ ] Modified Web platform tests (link to pull request) - 

Implementation commitment:

 * [ ] WebKit (https://bugs.webkit.org/show_bug.cgi?id=)
 * [ ] Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=)
 * [x] Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=1648944)


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  https://github.com/w3c/selection-api/pull/141

-- Commit Summary --

  * Fire selectionchange on input/textarea elements

-- File Changes --

    M index.html (7)

-- Patch Links --

https://github.com/w3c/selection-api/pull/141.patch
https://github.com/w3c/selection-api/pull/141.diff

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