* Add a `slimDOM` option to strip out unnecessary parts of the DOM in terms of replay
- <script> tags in the <head> take up unnecessary storage space and are often injected semi randomly to become a source of unnecessary variation between recordings of the same thing
- comment tags can be stripped out without affecting display
- future: this option could also turn on more aggressive stripping, e.g. elements that are hidden by CSS (assuming we can handle them becoming visible after mutation events)
* Mark nodes ignored due to slimDOM option, so that they can also be ignored by the mutation observer in rrweb
* Introducing the `ignored` attribute violates the `serializedNodeWithId` type
* slimDOM: Strip out whitespace nodes from <head> element as they have no effect but take up space
- these would otherwise have to be merged after <script> elements are removed; for statcounter usecase, removing
<script> elements is no good if there is still a trace of their presence due to the white space (and hence a variant <head> node is still produced)
- I explored a more radical stripping of all white space nodes, but there is a problem if parent node is <pre> or otherwise rendered with `white-space: pre` and similar.
detecting applied styles with getComputedStyle would be very expensive (I haven't measured it though)
* Export IGNORED_NODE as a constant instead of relying on the hard-to-grok `-2`
* Remove <link rel=preload as=script> which are similarly as useless as <script> tags
* Make slimDOM configurable with the expecations that `slimDOMOptions: true` will only enable non-destructive options (so not all options may be turned on)
* Expand slimDOM to add options to remove more elements from the <head> that should not be necessary in the replayer context
Also move the addHoverClass implementation into the rebuild stage.
So if there is still some corner case we have not handled, it will
only affect the replayer part of rrweb.
