Change Selection to be associated with a composed live range

[dizhang168]

Note: This PR is not ready to merge, but can reviewed. It depends on the definitions "composed live range" and "cached live range", being specced at: whatwg/dom#1342
The Editing WG issue discussing this change is: #2

To summarize, this spec PR changes:

1. Each selection can be associated with a single composed live range.
   Most of the existing Selection API functions will refer to the composed live range's cached live range to make sure we are backward compatible. However, for getComposedRanges(), we will refer to the composed live range's endpoints directly. This includes updating definitions for anchor/focus, getRangeAt(0), addRange(), removeRange(), etc.

2. Add "reset the range" algorithm
   We add a new "reset the range" of this selection algorithm, which is called by collapse(), collapseToStart(), collapseToEnd(), extend(), setBaseAndExtent(), selectAllChildren(). This will make sure to create and set the composed live range and its cached live range, and call {{Range}}'s "set the start/end" algorithm.

For normative changes, the following tasks have been completed:

• Editing WG resolution on the proposed changes, with at least two implementers participating and not objecting:

  • WebKit
  • Chromium
  • Gecko

• For browsers that are shipping the feature, implementation bugs are filed for the proposed changes (link to bug, or write "Not Implementing"):

  • WebKit (https://bugs.webkit.org/show_bug.cgi?id=)
  • Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=)
  • Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=)

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Preview | Diff

[dizhang168]

Here, the {{Range}} should refer to the Range interface defined in [dom]. However, it keeps pointing to range, which I redefined in this PR. How can I overwrite this behavior?

[domfarolino]

I think I would just remove the dfn of range that you introduced here. We already know the type of "composed live range", once it's able to link to DOM.