Replace placeholder - first step insert on index 0

[tplevko]

fixes: #1962

Instead of appending the step instead of the placeholder - add it to index 0.
This prevents the mentioned issue, when there are other steps already defined in the route.

[codecov]

Codecov Report

Merging #2006 (3331edf) into main (48fa76c) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2006   +/-   ##
=======================================
  Coverage   64.85%   64.85%           
=======================================
  Files          87       87           
  Lines        2828     2828           
  Branches      651      651           
=======================================
  Hits         1834     1834           
  Misses        948      948           
  Partials       46       46           

Impacted Files	Coverage Δ
src/services/stepsService.ts	71.92% <100.00%> (ø)

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
2.9% Duplication