feat(envvars): substitute envvars similar to decK

[Tieske]

when loading files this now behaves identical to deck.

[Tieske]

Note: the code is actually from decK, identical.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 59 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.22%. Comparing base (87f7094) to head (a82ac91).
⚠️ Report is 54 commits behind head on main.

Files with missing lines	Patch %	Lines
filebasics/template.go	0.00%	55 Missing ⚠️
filebasics/filebasics.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #182      +/-   ##
==========================================
- Coverage   67.76%   66.22%   -1.55%     
==========================================
  Files          23       24       +1     
  Lines        2491     2549      +58     
==========================================
  Hits         1688     1688              
- Misses        643      701      +58     
  Partials      160      160              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.