[toc] Performance Improvements

[Quang Phan]

Context

As toc@2.0.0 was released, almost everything was re-implemented from the ground up. This issue lists potential performance improvement

Todos

• reduce to use only one IntersectionObserver instead of one for each matching elements <- 60c2029
• run observe operations async <- a8cd5c2
• use simple callback instead of store for tracking active element <- d04d44e