Add eslint type assertions rule

Fixes #3359

src/browser/renderer/atlas/CharAtlasUtils.ts

@@ -10,7 +10,7 @@ import { ITerminalOptions } from 'common/services/Services';
 
 export function generateConfig(scaledCharWidth: number, scaledCharHeight: number, options: ITerminalOptions, colors: IColorSet): ICharAtlasConfig {
   // null out some fields that don't matter
-  const clonedColors = <IPartialColorSet>{
+  const clonedColors: IPartialColorSet = {
     foreground: colors.foreground,
     background: colors.background,
     cursor: undefined,

src/browser/renderer/dom/DomRenderer.ts

@@ -387,7 +387,7 @@ export class DomRenderer extends Disposable implements IRenderer {
       if (!row) {
         return;
       }
-      const span = <HTMLElement>row.children[x];
+      const span = row.children[x] as HTMLElement;
       if (span) {
         span.style.textDecoration = enabled ? 'underline' : 'none';
       }