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/utils/strings.ts

@@ -55,7 +55,7 @@ export async function copyPlaintext(text: string): Promise<boolean> {
     return false;
 }
 
-export function selectText(target: Element) {
+export function selectText(target: Element): void {
     const range = document.createRange();
     range.selectNodeContents(target);