Add eslint type assertions rule

Fixes #3359

src/browser/public/AddonManager.ts

@@ -31,7 +31,7 @@ export class AddonManager implements IDisposable {
     };
     this._addons.push(loadedAddon);
     instance.dispose = () => this._wrappedAddonDispose(loadedAddon);
-    instance.activate(<any>terminal);
+    instance.activate(terminal as any);
   }
 
   private _wrappedAddonDispose(loadedAddon: ILoadedAddon): void {

src/browser/public/Terminal.ts

@@ -280,7 +280,7 @@ class BufferLineApiView implements IBufferLineApi {
     }
 
     if (cell) {
-      this._line.loadCell(x, <ICellData>cell);
+      this._line.loadCell(x, cell as ICellData);
       return cell;
     }
     return this._line.loadCell(x, new CellData());