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/accessibility/RovingTabIndex.tsx

@@ -87,7 +87,7 @@ interface IAction {
     };
 }
 
-export const reducer = (state: IState, action: IAction) => {
+export const reducer: Reducer<IState, IAction> = (state: IState, action: IAction) => {
     switch (action.type) {
         case Type.Register: {
             if (!state.activeRef) {

src/accessibility/Toolbar.tsx

@@ -26,7 +26,7 @@ interface IProps extends Omit<React.HTMLProps<HTMLDivElement>, "onKeyDown"> {}
 // https://www.w3.org/TR/wai-aria-practices-1.1/#toolbar
 // All buttons passed in children must use RovingTabIndex to set `onFocus`, `isActive`, `ref`
 const Toolbar: React.FC<IProps> = ({ children, ...props }) => {
-    const onKeyDown = (ev: React.KeyboardEvent) => {
+    const onKeyDown = (ev: React.KeyboardEvent): void => {
         const target = ev.target as HTMLElement;
         // Don't interfere with input default keydown behaviour
         if (target.tagName === "INPUT") return;

src/accessibility/context_menu/StyledMenuItemCheckbox.tsx

@@ -33,7 +33,7 @@ interface IProps extends React.ComponentProps<typeof StyledCheckbox> {
 export const StyledMenuItemCheckbox: React.FC<IProps> = ({ children, label, onChange, onClose, ...props }) => {
     const [onFocus, isActive, ref] = useRovingTabIndex<HTMLInputElement>();
 
-    const onKeyDown = (e: React.KeyboardEvent) => {
+    const onKeyDown = (e: React.KeyboardEvent): void => {
         let handled = true;
         const action = getKeyBindingsManager().getAccessibilityAction(e);
 
@@ -55,7 +55,7 @@ export const StyledMenuItemCheckbox: React.FC<IProps> = ({ children, label, onCh
             e.preventDefault();
         }
     };
-    const onKeyUp = (e: React.KeyboardEvent) => {
+    const onKeyUp = (e: React.KeyboardEvent): void => {
         const action = getKeyBindingsManager().getAccessibilityAction(e);
         switch (action) {
             case KeyBindingAction.Space:

src/accessibility/context_menu/StyledMenuItemRadio.tsx

@@ -33,7 +33,7 @@ interface IProps extends React.ComponentProps<typeof StyledRadioButton> {
 export const StyledMenuItemRadio: React.FC<IProps> = ({ children, label, onChange, onClose, ...props }) => {
     const [onFocus, isActive, ref] = useRovingTabIndex<HTMLInputElement>();
 
-    const onKeyDown = (e: React.KeyboardEvent) => {
+    const onKeyDown = (e: React.KeyboardEvent): void => {
         let handled = true;
         const action = getKeyBindingsManager().getAccessibilityAction(e);
 
@@ -55,7 +55,7 @@ export const StyledMenuItemRadio: React.FC<IProps> = ({ children, label, onChang
             e.preventDefault();
         }
     };
-    const onKeyUp = (e: React.KeyboardEvent) => {
+    const onKeyUp = (e: React.KeyboardEvent): void => {
         const action = getKeyBindingsManager().getAccessibilityAction(e);
         switch (action) {
             case KeyBindingAction.Enter: