Single style capture (#1437)

Support a contrived/rare case where a <style> element has multiple text node children (this is usually only possible to recreate via javascript append) ... this PR fixes cases where there are subsequent text mutations to these nodes; previously these would have been lost

* In this scenario, a new CSS comment may now be inserted into the captured `_cssText` for a <style> element to show where it should be broken up into text elements upon replay: `/* rr_split */`
* The new 'can record and replay style mutations' test is the principal way to the problematic scenarios, and is a detailed 'catch-all' test with many checks to cover most of the ways things can fail
* There are new tests for splitting/rebuilding the css using the rr_split marker
* The prior 'dynamic stylesheet' route is now the main route for serializing a stylesheet; dynamic stylesheet were missed out in #1533 but that case is now covered with this PR

This PR was originally extracted from #1475 so the  initial motivation was to change the approach on stringifying <style> elements to do so in a single place.  This is also the motivating factor for always serializing <style> elements via the `_cssText` attribute rather than in it's childNodes; in #1475 we will be delaying populating `_cssText` for performance and instead recorrding them as assets.

Thanks for the detailed review to  Justin Halsall <Juice10@users.noreply.github.com> & Yun Feng <https://github.com/YunFeng0817>

packages/rrweb/package.json

@@ -5,7 +5,8 @@
   "scripts": {
     "prepare": "npm run prepack",
     "prepack": "npm run build",
-    "retest": "vitest run --exclude test/benchmark",
+    "retest": "cross-env PUPPETEER_HEADLESS=true yarn retest:headful",
+    "retest:headful": "vitest run --exclude test/benchmark",
     "build-and-test": "yarn build && yarn retest",
     "test:headless": "cross-env PUPPETEER_HEADLESS=true yarn build-and-test",
     "test:headful": "cross-env PUPPETEER_HEADLESS=false yarn build-and-test",