fix: Add stub for files event to fix psalm error

[susnux]

Fixing:

ERROR: InvalidArgument - lib/AppInfo/Application.php:139:13 - Incompatible types found for T (OCA\Files\Event\LoadSidebar|OCP\EventDispatcher\Event is not in OCP\EventDispatcher\Event) (see https://psalm.dev/004)
                $context->registerEventListener(LoadSidebar::class, LoadSidebarScripts::class);

[susnux]

Cypress unrelated, seems to be broken on master branch.
So guess it can be merged by someone with permissions :)

[miaulalala]

I can't. @artonge can you?

[artonge]

No

[susnux]

In this case I try to fix Cypress as well

[github-actions]

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