Refactor project (standard layout, split up Knit from Gradle plugin, buildSrc convention plugins)

[aSemy]

Restructure project, and improve Gradle compatibility

• split Gradle plugin into separate subproject
• move 'core' knit code into separate subproject
• Create buildSrc convention plugins #50
  • [x] finish publishing convention plugins
• Update Gradle plugin to use Provider API #47
• Restructure project to use standard layout #48
• Implement Gradle version catalog
• Move testdata into test/resources (I found it difficult to figure out how the testdata dir was retrieved dynamically - I found it much easier when it was within the resources dir)

[qwwdfsad]

Hi, for now I'm the only active maintainer, and I'm stretched a bit too thin, so expect a delayed review here. Sorry for the inconvenience!

[aSemy]

No problem! I'm mostly refactoring so I can make something work for my own project, and sharing my work in case you'd also like it.