Serializable Groovy, Java, Kotlin, and Scala steps

[Goooler]

Refs #1274 (comment).

[nedtwigg]

Amazing @Goooler, thanks so much! I'm gonna pull this locally, do some testing, and then merge. Sorry it sat for so long.

[nedtwigg]

The intent here is to be a marker interface?

[Goooler]

Yeah.

[jbduncan]

@Goooler My YAGNI senses are tingling! I'm wondering if you can clarify why this interface was introduced, and if we can get away with inlining Serializable instead?

Otherwise, this all LGTM. 👍

[Goooler]

I was thinking that we can do some checks based on a common step type instead of names here

spotless/testlib/src/main/java/com/diffplug/spotless/StepHarnessBase.java

Lines 52 to 55 in 224f8f9

	} else if (Set.of("black", "buf", "clang", "ktlint", "ktfmt", "scalafmt", "palantir-java-format", "google-java-format",
	"removeUnusedImports", "cleanthat", "No line break between type annotation and type",
	"importOrder", "Remove unnecessary semicolons").contains(onlyStepName)) {
	supportsRoundTrip = true;

[nedtwigg]

That's a good idea, but it's hard to get to that state. I figured it was probably meant as a testing thing, we can remove it when we finish #1274

[jbduncan]

Sounds good to me either way! 👍

[nedtwigg]

I added some comments in

• Comments on #2033 Goooler/spotless#16

If you run the first commit, you'll see the tests fail, and then pass in the second commit. Wanna push those commits into this PR and then I'll press merge?

[Goooler]

Thanks for your feedback!