refactor(core): avoid forEach and increase test coverage for forge-config

[malept]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

Also fixes an edge case with config templates.

[codecov]

Codecov Report

Merging #1395 into master will increase coverage by 2.57%.
The diff coverage is 90.9%.

@@            Coverage Diff             @@
##           master    #1395      +/-   ##
==========================================
+ Coverage    67.6%   70.17%   +2.57%     
==========================================
  Files          72       72              
  Lines        2099     2099              
  Branches      389      389              
==========================================
+ Hits         1419     1473      +54     
+ Misses        525      452      -73     
- Partials      155      174      +19

Impacted Files	Coverage Δ
packages/api/core/src/util/forge-config.ts	80.3% <90.9%> (+12.12%)	⬆️
packages/maker/flatpak/src/MakerFlatpak.ts	78.26% <0%> (-8.7%)	⬇️
packages/api/core/src/api/import.ts	58.82% <0%> (ø)	⬆️
packages/maker/base/src/Maker.ts	72.97% <0%> (+2.7%)	⬆️
packages/maker/dmg/src/MakerDMG.ts	100% <0%> (+5.55%)	⬆️
packages/api/core/src/api/make.ts	70.78% <0%> (+5.61%)	⬆️
packages/maker/zip/src/MakerZIP.ts	100% <0%> (+9.09%)	⬆️
packages/maker/wix/src/util/author-name.ts	90% <0%> (+10%)	⬆️
packages/maker/appx/src/util/author-name.ts	90% <0%> (+10%)	⬆️
... and 3 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2dc0401...932a383. Read the comment docs.