Fix Luacheck issues

[victorpopkov]

Verify

• I understand that if I ignore these instructions, my issue may be closed without review.
• I have made sure that there are no similar opened issues.
• I have made sure that the mod version matches the latest release.

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

After closing both #6 and #8, all warnings related to Luacheck should get fixed. It doesn't have auto-fixing mode, so it will be easier to deal with it in the end.

Describe the solution you'd like

Fix Luacheck issues.

Describe alternatives you've considered

If applicable, add a clear and concise description of any alternative solutions or features you've considered.

Additional context

If applicable, add any other context or screenshots about the feature request here.