perf: coerce blockSelector and blockClass to a single concept and reduce branching

[JonasBa]

this builds on top of https://github.com/rrweb-io/rrweb/pull/1536, the idea is to create a single blocking selector and remove some runtime checking and branching inside isBlocked

This is a stacked PR btw, once upstream is merged I will rebase so it'll only contain the related changes