Don't include dom typings in extensions (#89942)

**Problem**
All of our extensions currently are built using the dom typings. This can lead to runtime errors if you mistakenly use `window` or similar

**Fix**
Exclude the dom typings from compile. Then explicitly import the node types for `URL` and `TextEncoder`

extensions/markdown-language-features/preview-src/tsconfig.json

@@ -2,6 +2,11 @@
 	"extends": "../../shared.tsconfig.json",
 	"compilerOptions": {
 		"outDir": "./dist/",
-		"jsx": "react"
+		"jsx": "react",
+		"lib": [
+			"es2018",
+			"DOM",
+			"DOM.Iterable"
+		]
 	}
 }