[dom] Remove feature detection from add() and add supports() (#123)

This follows up on the discussion we had in https://github.com/whatwg/dom/pull/114 regarding feature detection for DOMTokenLists:

* It removes references to "validation steps" from `add()`, `toggle()`, `replace()` and object creation.
* It adds `supports()` as discussed.
* When the associated attribute has no supported tokens, `TypeError` is thrown (by the "validation steps")

/cc @zcorpan @domenic @annevk 
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/dom/pull/123

-- Commit Summary --

  * Remove feature detection from add() and add supports()

-- File Changes --

    M dom.bs (49)
    M dom.html (72)

-- Patch Links --

https://github.com/whatwg/dom/pull/123.patch
https://github.com/whatwg/dom/pull/123.diff

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/dom/pull/123