implement `transform-origin`

[adamwong246]

fixes #156

Implements the transform-origin property as documented in the following sources:

• https://developer.mozilla.org/en-US/docs/Web/CSS/transform-origin
• https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/transform-origin
• https://drafts.csswg.org/css-transforms/#transform-origin-property
• https://drafts.csswg.org/css-transforms/#transformation-matrix-computation

Our team needed this feature, so I hope you'll find it useful as well. The work is ATM incomplete but suffices for our team. I'll be doing the pro-bono work needed to get this PR merged back upstream- let me know what else is required. A very narrow test has been made and the results are checked in as a pdf matching the official test image

Work left to do:

• The property can only be defined as 2 absolute or 2 percentage values, not with single values or with keywords.
• There's a problem where the classlist is returned as a list of strings, rather than a "live DOMTokenList collection". I suspect it is a web vs node thing but I need to investigate.

Also I added node_modules to the .gitignore ;-)