fix(dom): Implement getElementsByClassName as specified

[ChALkeR]

Reimplements Document.getElementsByClassName without using a dynamically generated regex.

1. Unlike CSS grammar, there seems to be no limitation on which classnames are valid in HTML/DOM (given that they don't contain space chars). That is, [ seems to be a valid classname, which was broken in this impl as it was fed into a regexp unescaped and caused xmldom to throw.
2. Whitespace processing should be now correct. \s in JS regexps includes more chars than space-separated tokens of html.
3. This now should support a list of space-separated classnames as an argument. Previously, that was broken and order-dependent.
4. This prevents causing a ReDoS by passing in exponential regexps as strings to getElementsByClassName() method,
   e.g. new RegExp("(^|\\s)" + '(((a||||)+)+)+' + "(\\s|$)").test('aaaaab').

Refs:

1. https://dom.spec.whatwg.org/#dom-document-getelementsbyclassname
2. https://dom.spec.whatwg.org/#dom-element-getelementsbyclassname
3. https://dom.spec.whatwg.org/#concept-getelementsbyclassname
4. https://www.w3.org/TR/html52/dom.html#element-attrdef-global-class
5. https://www.w3.org/TR/html52/infrastructure.html#set-of-space-separated-tokens
6. Implement Document.getElementsByClassName #24
7. Fix #24 Implement Document.getElementsByClassName #25

Note: I couldn't find the tests for this and am unsure how to test this. cc @codler perhaps — could you confirm this would work?
Seems to work for me with simple playground tests.

[karfau]

Thank you for discovering this and for your contribution.
I added some suggestions and recommendations.
If you don't have the capacity or knowledge, to do what I'm asking for, feel free to ask any questions for support, or let us know if/when it's beyond your capacity.

[karfau]

This PR will also have an impact on #100 since that one tries to add the ability to also call getElementsByClassName on an Element.

[karfau]

@ChALkeR please let us know if you plan to continue to work on this on your own, thx

[karfau]

I resolved all the remarks that I added earlier. Thanks again for your contribution.

I needed to force push the rebased branch, other wise github was showing changes to some 30 files and conflicts.

I took the liberty to rename the PR and do a minor tweak to the PR description.