eslint: Enable more lints from @wordpress/eslint-plugin (#39000)

- no-alert: Enabled for new instances. Existing instances are ignored,
  some with todo comments.
- no-bitwise: Most existing instances were intentional. Replaced one
  `| 0` with `Math.floor()`.
- no-fallthrough: One error detected and fixed. Added fall-through
  comments to the rest.
- no-useless-computed-key: Auto-fixed them all.
- no-useless-return: Auto-fixed, then had to clean up some places where
  it left empty blocks or otherwise exposed weird code flows.

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/10508553954

Upstream-Ref: Automattic/jetpack@befb138

vendor/automattic/jetpack-mu-wpcom/src/common/tour-kit/utils/live-resize-modifier.tsx

@@ -53,9 +53,7 @@ export const liveResizeModifier: liveResizeModifierFactory = (
 	name: 'liveResizeModifier',
 	enabled: true,
 	phase: 'main',
-	fn: () => {
-		return;
-	},
+	fn: () => {},
 	effect: arg0 => {
 		try {
 			const { state, instance } = arg0 as ModifierArgumentsWithObserversProp; // augment types here because we are mutating the properties on the argument that is passed in