Add Element.Matches and Element.Closest

[inkeliz]

This change add support for:

element . matches(selectors)
Returns true if matching selectors against element’s root yields element, and false otherwise.

element . closest(selectors)
Returns the first (starting at element) inclusive ancestor that matches selectors, and null otherwise.

[dmitshur]

Sorry about the delay.

This PR currently includes 4 commits, but the intention appears to have been to include only commit
ec897f4. The rest are not related and are a part of PR #66.

I'll push to your matches-closest branch to resolve drop the unrelated commits 6a71e80, 52bf9bb, and 2979c34.

[dmitshur]

LGTM. Thanks!

[dmitshur]

Adding methods to an exported interface is technically a breaking API change (if anyone happened to implement this interface externally, their implementation would no longer satisfy it).

We have no choice but to allow this, because the Element interface is meant to map to the DOM Element interface, and the policy of this package allows for such API changes.