Add `classList` + functions of `DOMTokenList`

[sectore]

• All the following functions of DOMTokenList are added and supported by all modern browsers (see https://developer.mozilla.org/en-US/docs/Web/API/DOMTokenList):

  • add
  • remove
  • toggle
  • contains
  • item

• incl. tests (depends on PR #105 )

[sectore]

@garyb No problem, latest commits ⬆️ come with these fixes (rebase + remove dead code + rearrange arguments).

[garyb]

Thank you!