fast-forward implementation v2: virtual dom optimization

[YunFeng0817]

Motivation

In PR #241, we applied a virtual parent optimization to the fast-forward playback where we got a 10 times performance improvement. But this approach still has some unavoidable defects, e.g. #550, #529, #827.

The cause and solutions of these issues have been explained here: #548 (comment). I believe the virtual dom could be a good solution to solve these issues and it can also bring an approximately 2.5x performance boost.

changes

This PR has a huge amount of code changes and I'll explain them to help understand.

1. In rrdom: an ad-hoc DOM for rrweb session data #419 and PR rrdom #613, we designed and implemented an ad-hoc DOM called 'rrdom' as an environment to replay the session. In this PR, I make some changes based on it and make it a browser runtime specially designed for virtual-dom. The feature of this runtime is that it is slim and only covers the dom operations used in the rrweb replayer. In other words, the virtual dom is implemented with rrdom.
2. The main mechanism of virtual dom optimization is described below. At the start of fast-forward process, a virtual dom tree is created which is a clone of the real dom tree. More precisely, the virtual dom is created right before the replayer starts to process the first node mutation event. After that, any node mutations are applied to the virtual dom tree. After all incremental events have been applied synchronously, the replayer goes into the flush stage. At this stage, the replayer executes a diff algorithm between the virtual-dom and the real dom, and applies all mutations to the real dom.
3. PR fast-forward implementation v1 #241 uses a data structure called treeIndex to improve the performance of applying input events and scroll events. This PR takes a similar approach to replace it. The key idea is to delay the handling of these types of events until the flush stage. Input events, scroll events, canvas mutations, and CSS style mutations are stored on their target virtual nodes at first. These data will be processed only after the real dom is updated in the flush stage. BTW, the virtual stylesheet created by Buffer modifications to virtual stylesheets #618 is also integrated into the virtual dom in this PR.
4. To improve the quality and reliability of this PR, the rrdom package has reached a high test coverage listed below:

File	% Stmts	% Branch	% Funcs	% Lines
All files	97.35	94.56	97.28	97.35
diff.ts	92.34	91.33	91.66	92.15
document-nodejs.ts	100	100	100	100
document.ts	100	100	100	100
polyfill.ts	95.83	100	90.9	95.34
style.ts	100	83.33	100	100
virtual-dom.ts	97.6	91.78	89.28	98.37

Several integration test cases are also added to cover some types of replay scenes.

Some minor changes may relate to these PRs: #427, #481, #662, #666, #671, #756.