Add file extensions to all imports

[st3iny]

Is your feature request related to a problem? Please describe.

After #4173 eslint will generate a lot of warning about missing extensions. It's even considered to promote this rule to an error in the future (ref nextcloud-libraries/eslint-config#292).

Describe the solution you'd like

Refactor all imports to use file endings. This can probably be automated.

Describe alternatives you've considered

No response

Additional context

No response