feat(richtext-lexical)!: upgrade lexical from 0.15.0 to 0.16.0 and port over relevant playground changes (#6620)

**BREAKING:**
- This upgrades the required version of lexical from 0.15.0 to 0.16.0.
If you are using lexical directly in your project, possibly due to
custom features, there might be breaking changes for you. Please consult
the lexical 0.16.0 changelog:
https://github.com/facebook/lexical/releases/tag/v0.16.0

test/package.json

@@ -12,8 +12,8 @@
     "typecheck": "pnpm turbo build --filter test && tsc --project tsconfig.typecheck.json"
   },
   "devDependencies": {
-    "@lexical/headless": "0.15.0",
-    "@lexical/markdown": "0.15.0",
+    "@lexical/headless": "0.16.0",
+    "@lexical/markdown": "0.16.0",
     "@payloadcms/db-mongodb": "workspace:*",
     "@payloadcms/db-postgres": "workspace:*",
     "@payloadcms/email-nodemailer": "workspace:*",