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-history/package.json

@@ -21,7 +21,7 @@
     "lodash": "^4.17.21",
     "slate": "^0.81.0",
     "slate-hyperscript": "^0.77.0",
-    "source-map-loader": "^0.2.4"
+    "source-map-loader": "^4.0.0"
   },
   "peerDependencies": {
     "slate": ">=0.65.3"

packages/slate-history/src/history-editor.ts

@@ -19,6 +19,7 @@ export interface HistoryEditor extends BaseEditor {
   redo: () => void
 }
 
+// eslint-disable-next-line no-redeclare
 export const HistoryEditor = {
   /**
    * Check if a value is a `HistoryEditor` object.

packages/slate-history/src/history.ts

@@ -11,6 +11,7 @@ export interface History {
   undos: Operation[][]
 }
 
+// eslint-disable-next-line no-redeclare
 export const History = {
   /**
    * Check if a value is a `History` object.