Upgrade next.js to 12 and related packages (#5042)

* Upgrade next.js and source-map-loader

* Add changeset

* Upgrade eslint and typescript

* improve reliability of iframe test

* fix lint

packages/slate-react/package.json

@@ -35,7 +35,7 @@
     "react-test-renderer": ">=16.8.0",
     "slate": "^0.81.0",
     "slate-hyperscript": "^0.77.0",
-    "source-map-loader": "^0.2.4"
+    "source-map-loader": "^4.0.0"
   },
   "peerDependencies": {
     "react": ">=16.8.0",

packages/slate-react/src/components/editable.tsx

@@ -1479,6 +1479,7 @@ const defaultScrollSelectionIntoView = (
     scrollIntoView(leafEl, {
       scrollMode: 'if-needed',
     })
+    // @ts-expect-error an unorthodox delete D:
     delete leafEl.getBoundingClientRect
   }
 }

packages/slate-react/src/plugin/react-editor.ts

@@ -51,6 +51,7 @@ export interface ReactEditor extends BaseEditor {
   hasRange: (editor: ReactEditor, range: Range) => boolean
 }
 
+// eslint-disable-next-line no-redeclare
 export const ReactEditor = {
   /**
    * Check if the user is currently composing inside the editor.