Add JVM-based JSON formatter

[jamietanna]

It would be good to have a JVM-only version of JSON formatters, without needing to have prettier installed.

I'd be happy to contribute this, for Gradle - would it be best to raise a PR to add this formatter, or shall I create it as a separate project?

[nedtwigg]

JVM-only json formatter is very welcome! If you can implement it without any third-party dependencies, then it can live in lib like the sql formatter.

If it needs just third-party dependencies then it should live in lib-extra.

spotless/lib-extra/build.gradle

Lines 11 to 18 in 59173b0

	// misc useful utilities
	implementation "com.diffplug.durian:durian-core:${VER_DURIAN}"
	implementation "com.diffplug.durian:durian-collect:${VER_DURIAN}"
	// needed by GitAttributesLineEndings
	implementation "org.eclipse.jgit:org.eclipse.jgit:${VER_JGIT}"
	implementation "com.googlecode.concurrent-trees:concurrent-trees:2.6.1"
	// used for xml parsing in EclipseFormatter
	implementation "org.codehaus.groovy:groovy-xml:3.0.7"

If the dependencies you need cause a lot of conflicts (ala #587), then we can always pull it out into a third-party lib later on.