Add: Extension function on 'Project' to easily configure the 'rootCoverage' extension

[Rolf-Smit]

This is especially helpful when using Kotlin (.kts) as language for build files.

Closes: #112

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 83.47%. Comparing base (29beee5) to head (269dce9).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...plugin/rootcoverage/RootCoveragePluginExtension.kt	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #113      +/-   ##
============================================
+ Coverage     81.55%   83.47%   +1.91%     
  Complexity       48       48              
============================================
  Files             8        8              
  Lines           206      242      +36     
  Branches         30       30              
============================================
+ Hits            168      202      +34     
- Misses           11       13       +2     
  Partials         27       27              

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