Enable @typescript-eslint/explicit-function-return-type in /src (#9788)

* Enable `@typescript-eslint/explicit-member-accessibility` on /src

* Prettier

* Enable `@typescript-eslint/explicit-function-return-type` in /src

* Fix types

* tsc strict fixes

* Delint

* Fix test

* Fix bad merge

src/@types/diff-dom.d.ts

@@ -20,10 +20,10 @@ declare module "diff-dom" {
         name: string;
         text?: string;
         route: number[];
-        value: string;
-        element: unknown;
-        oldValue: string;
-        newValue: string;
+        value: HTMLElement | string;
+        element: HTMLElement | string;
+        oldValue: HTMLElement | string;
+        newValue: HTMLElement | string;
     }
 
     interface IOpts {}

src/@types/polyfill.ts

@@ -15,7 +15,7 @@ limitations under the License.
 */
 
 // This is intended to fix re-resizer because of its unguarded `instanceof TouchEvent` checks.
-export function polyfillTouchEvent() {
+export function polyfillTouchEvent(): void {
     // Firefox doesn't have touch events without touch devices being present, so create a fake
     // one we can rely on lying about.
     if (!window.TouchEvent) {