feat: Dark mode support for iconpicker

[fsbraun]

Description

This PR adds dark mode support (with django CMS 3.11+) for the icon picker. Also, it runs tests against Django 4.0.

It adds a .nvmrc for version 8 to be able to run gulp. Finally, it prepares to be tested against django CMS 4.

Checklist

• [x] I have opened this pull request against master
• [x] I have added or modified the tests when changing logic
• [x] I have followed the conventional commits guidelines to add meaningful information into the changelog
• [x] I have read the contribution guidelines and I have joined #workgroup-pr-review on Slack to find a “pr review buddy” who is going to review my pull request.

[codecov[bot]]

Codecov Report

Merging #46 (1a6d935) into master (20efce5) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #46   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            8         9    +1     
  Lines          123       128    +5     
  Branches        20        21    +1     
=========================================
+ Hits           123       128    +5     

Impacted Files	Coverage Δ
...s_icon/migrations/0003_alter_icon_cmsplugin_ptr.py	100.00% <100.00%> (ø)

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