Migrate to `gradle.kts` (Gradle Kotlin DSL)

[will-molloy]

A bit painful!

• IntelliJ doesn't give syntax highlighting until file is valid
• Documentation sucks... have to search GitHub to see how other people use plugins etc.
• Some plugins broken (test-sets), so used workaround

Closes #232

[codecov[bot]]

Codecov Report

Merging #234 (3768981) into main (3d6fb33) will not change coverage. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##                main      #234   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity         2         2           
===========================================
  Files              1         1           
  Lines              4         4           
===========================================
  Hits               4         4           

Flag	Coverage Δ
integration-tests-macos-latest	?
integration-tests-ubuntu-latest	100.00% <100.00%> (ø)
integration-tests-windows-latest	100.00% <100.00%> (ø)
unit-tests-macos-latest	100.00% <100.00%> (ø)
unit-tests-ubuntu-latest	100.00% <100.00%> (ø)
unit-tests-windows-latest	100.00% <100.00%> (ø)

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Impacted Files	Coverage Δ
...world/src/main/java/com/willmolloy/HelloWorld.java	100.00% <100.00%> (ø)