Replace groovy script with FreeMarker template engine

[mlvandijk]

🤔 What's changed?

Replace groovy scripts with FreeMarker templates to generate code.

⚡️ What's your motivation?

Fixes #2648

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, documentation etc. without changing behaviour)

♻️ Anything particular you want feedback on?

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• I've changed the behaviour of the code
  • I have added/updated tests to cover my changes.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• Users should know about my change
  • I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

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[codecov]

Codecov Report

Merging #2669 (3aea8ab) into main (5567a43) will increase coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 3aea8ab differs from pull request most recent head 16bf7f5. Consider uploading reports for the commit 16bf7f5 to get more accurate results

@@            Coverage Diff            @@
##               main    #2669   +/-   ##
=========================================
  Coverage     84.68%   84.68%           
+ Complexity     2682     2680    -2     
=========================================
  Files           322      322           
  Lines          9449     9450    +1     
  Branches        899      899           
=========================================
+ Hits           8002     8003    +1     
  Misses         1120     1120           
  Partials        327      327           

Impacted Files	Coverage Δ
...in/java/io/cucumber/spring/TestContextAdaptor.java	93.80% <0.00%> (ø)
...rc/main/java/io/cucumber/spring/SpringFactory.java	87.50% <0.00%> (+0.40%)	⬆️

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[mpkorstanje]

Nice! But it didn't work on my machine because of relative paths. I guess. I've pushed some fixes for that to the cucumber-java part.

Also put the code gen code into src/codegen/java instead. This is then compiled to target/codegen-classes to avoid packaging the code gen code in the jar build with mvn package.

For now lets polish cucumber-java that before we copy it over to cucumber-java8. It would be good to keep in mind that this will be copied over. You can do this by extracting all the configuration constants to the top of the file and/or program arguments.