[Proposal] Deprecate meta-events

[marcospassos]

I want to propose deprecating the meta-attributes such as isSvg, isStyle, etc. The main reason behind this proposal is that it makes it easier for those migrating from another recording tool to RRWeb. I believe that these meta-attributes can be inferred at replay-time by inspecting the parent nodes in a backward-compatible manner.

For <svg> tags, the rebuild logic checks whether it is an instance of an SVGElement, and then create the element with the SVG namespace. I believe, however, it's not strictly needed: for every element C that is a child of another element P, if P is an SVGElement then C is also an SVGElement. That said, during replay, we can just check if the parent element is an instance of SVGElement instead of looking at the isSvg flag.
We can use the same logic for the isStyle, but checking if the parent is an <style> element.