Added position to BuildConfigField to keep the insertion order

[gmazzo]

Fixes #52 allowing to keep the insertion order, by introducing a position field to BuildConfigField which defaults to the collection size.

[codecov]

Codecov Report

Merging #58 (73ffe6d) into main (7cb7079) will increase coverage by 0.07%.
The diff coverage is 87.50%.

@@             Coverage Diff              @@
##               main      #58      +/-   ##
============================================
+ Coverage     80.34%   80.42%   +0.07%     
- Complexity       53       55       +2     
============================================
  Files            15       15              
  Lines           229      235       +6     
  Branches         18       19       +1     
============================================
+ Hits            184      189       +5     
  Misses           32       32              
- Partials         13       14       +1     

Impacted Files	Coverage Δ
...om/github/gmazzo/gradle/plugins/BuildConfigTask.kt	78.26% <80.00%> (-1.74%)	⬇️
...le/plugins/internal/DefaultBuildConfigClassSpec.kt	92.30% <100.00%> (+2.30%)	⬆️

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