fix: nested stylesheets should have absolute URLs (#1533)

* Replace relative URLs with absolute URLs when stringifying stylesheets

* Add test to show desired behavior for imported stylesheets from seperate directory

* Rename `absoluteToStylesheet` to `absolutifyURLs` and call it once after stringifying imported stylesheet

* Don't create the intermediary array of the spread operator

* Formalize that `stringifyRule` should expect a sheet href

* Ensure a <style> element can also import and gets it's url absolutized

* Handle case where non imported stylesheet has relative urls that need to be absolutified

* Clarify in test files where jpegs are expected to appear in absolutified urls

* Move absolutifyURLs call for import rules out of trycatch

* Add a benchmarking test for stringifyStylesheet

* Avoid the duplication on how to fall back

---------

Co-authored-by: Eoghan Murray <eoghan@getthere.ie>
Co-authored-by: eoghanmurray <eoghanmurray@users.noreply.github.com>

packages/rrweb-snapshot/test/css/style-with-import.css

@@ -1,2 +1,3 @@
 @import '//fonts.googleapis.com/css2?family=Open+Sans:wght@400;600;700&family=Roboto:wght@100;300;400;500;700&display=swap"';
 @import './style.css';
+@import '../alt-css/alt-style.css';

packages/rrweb-snapshot/test/css/style.css

@@ -1,11 +1,11 @@
 body {
   margin: 0;
-  background: url('../a.jpg');
+  background: url('../should-be-in-root-folder.jpg');
   border-image: url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" x="0px" y="0px" viewBox="0 0 256 256"><g><g><polygon points="79.093,0 48.907,30.187 146.72,128 48.907,225.813 79.093,256 207.093,128"/></g></g></svg>');
 }
 p {
   color: red;
-  background: url('./b.jpg');
+  background: url('./should-be-in-css-folder.jpg');
 }
 body > p {
   color: yellow;