Demo: drop dangling astUpdateField reference

I deleted the astUpdateField StateField definition during the earlier
ESLint cleanup but missed the reference inside commonExtensions(), so
demo.ts shipped a ReferenceError at runtime: every editor instance
threw before its initial doc was applied, which is why the live demo
rendered empty editor panels with the samples dropdown populated but
no content. The TS plugin warned in red but the build didn't error,
so the broken bundle reached production.

Also tightens the ESLint config — flipping no-undef back on for examples/
catches this exact class of regression in CI.

Author: nixel2007

examples/demo.ts

@@ -249,7 +249,6 @@ function commonExtensions(): Extension[] {
     basicSetup,
     bsl(),
     astUpdater,
-    astUpdateField,
     ...(dark ? [oneDark] : [])
   ]
 }