Feat: Add support for replaying :defined pseudo-class of custom elements (#1155)

* Feat: Add support for replaying :defined pseudo-class of custom elements

* add isCustom flag to serialized elements

Applying Justin's review suggestion

* fix code lint error

* add custom element event

* fix: tests (#1348)

* Update packages/rrweb/src/record/observer.ts

* Update packages/rrweb/src/record/observer.ts

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Co-authored-by: Nafees Nehar <nafees87n@gmail.com>
Co-authored-by: Justin Halsall <Juice10@users.noreply.github.com>

packages/rrweb-snapshot/tsconfig.json

@@ -2,6 +2,7 @@
   "compilerOptions": {
     "composite": true,
     "module": "ESNext",
+    "target": "ES6",
     "moduleResolution": "Node",
     "noImplicitAny": true,
     "strictNullChecks": true,