chore: add envrc files

[limwa]

Adds .envrc files
Editors with compliant extensions will automatically use the flake.nix at the repository root to setup the developmennt environment.

Review checklist

• Terms and conditions reflect the current change
• Contains enough appropriate tests
• If aimed at production, writes a new summary in whatsnew/whatsnew-pt-PT
• Properly adds an entry in changelog.md with the change
• If PR includes UI updates/additions, its description has screenshots
• Behavior is as expected
• Clean, well-structured code

[limwa]

uni_lint package is being introduced in #1400.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 12%. Comparing base (b70f75b) to head (e5e4b87).
Report is 4 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #1409   +/-   ##
=======================================
  Coverage       12%     12%           
=======================================
  Files          264     264           
  Lines         7238    7238           
=======================================
  Hits           803     803           
  Misses        6435    6435